diff --git a/server/src/accounts.test.ts b/server/src/accounts.test.ts
new file mode 100644
index 0000000..431dd5e
--- /dev/null
+++ b/server/src/accounts.test.ts
@@ -0,0 +1,141 @@
+import { test, after } from "node:test";
+import assert from "node:assert/strict";
+
+/**
+ * inbuxa MA-B: more than one account signed in in one browser. The session
+ * cookie is the account in front; the others ride in `<name>_more`. Adding
+ * signs a second account in beside the first, switching swaps them, signing
+ * out ends only the one in front, and an organization that doesn't allow it
+ * keeps adding off.
+ */
+
+const PORT = 18801;
+process.env.MOCK_PORT = String(PORT);
+process.env.MOCK_USER = "first@example.com";
+process.env.MOCK_PASS = "first-password";
+process.env.MOCK_SECOND_USER = "second@example.com";
+process.env.MOCK_SECOND_PASS = "second-password";
+process.env.MAIL_SERVER_URL = `http://127.0.0.1:${PORT}`;
+process.env.APP_SECRET = "test-secret-for-accounts";
+
+const mock = await import("./mock/index.js");
+const { createApp } = await import("./app.js");
+const { config } = await import("./config.js");
+const { MAX_ACCOUNTS, parseOthers, serializeOthers } = await import("./accounts.js");
+
+const app = createApp();
+const FRONT = config.cookieName;
+const MORE = `${config.cookieName}_more`;
+
+after(() => {
+  (mock as { server?: { close(): void } }).server?.close();
+});
+
+/** A browser's cookie jar, as far as these two cookies go. */
+class Browser {
+  jar = new Map<string, string>();
+
+  private take(res: Response) {
+    for (const line of res.headers.getSetCookie()) {
+      const [pair, ...attrs] = line.split(";");
+      const at = pair!.indexOf("=");
+      const name = pair!.slice(0, at).trim();
+      const value = pair!.slice(at + 1).trim();
+      const expired = attrs.some((a) => /max-age=0/i.test(a) || /expires=thu, 01 jan 1970/i.test(a));
+      if (expired || !value) this.jar.delete(name);
+      else this.jar.set(name, value);
+    }
+  }
+
+  async call(path: string, init: { method?: string; body?: unknown } = {}): Promise<{ status: number; body: any }> {
+    const cookie = [...this.jar].map(([k, v]) => `${k}=${v}`).join("; ");
+    const res = await app.request(path, {
+      method: init.method ?? "GET",
+      headers: { "content-type": "application/json", "x-requested-with": "ihasmail", ...(cookie ? { cookie } : {}) },
+      ...(init.body !== undefined ? { body: JSON.stringify(init.body) } : {}),
+    });
+    this.take(res);
+    const text = await res.text();
+    return { status: res.status, body: text ? JSON.parse(text) : null };
+  }
+
+  signIn(username: string, password: string, add = false) {
+    return this.call("/api/auth/login", { method: "POST", body: { username, password, ...(add ? { add: true } : {}) } });
+  }
+
+  async accounts(): Promise<{ username: string; front: boolean; id: string }[]> {
+    const res = await this.call("/api/auth/accounts");
+    assert.equal(res.status, 200, JSON.stringify(res.body));
+    return res.body.accounts;
+  }
+}
+
+test("the cookie list keeps only well-formed session cookies, at most one fewer than the cap", () => {
+  const good = "abcdefghij.ABCDEFGHIJKLMN";
+  assert.deepEqual(parseOthers(`${good}~not a cookie~${good}`), [good]);
+  const many = Array.from({ length: 9 }, (_, i) => `abcdefgh${i}x.ABCDEFGHIJKLMN`);
+  assert.equal(parseOthers(many.join("~")).length, MAX_ACCOUNTS - 1);
+  assert.equal(serializeOthers(["bad", good]), good);
+});
+
+test("a second account joins the first, and switching swaps them", async () => {
+  const b = new Browser();
+  assert.equal((await b.signIn("first@example.com", "first-password")).status, 200);
+  assert.deepEqual((await b.accounts()).map((a) => a.username), ["first@example.com"]);
+  const first = b.jar.get(FRONT);
+
+  const added = await b.signIn("second@example.com", "second-password", true);
+  assert.equal(added.status, 200, JSON.stringify(added.body));
+  assert.equal(added.body.added, true);
+  assert.equal(b.jar.get(MORE), first, "the first account moved beside the new one");
+  let accounts = await b.accounts();
+  assert.deepEqual(accounts.map((a) => [a.username, a.front]), [["second@example.com", true], ["first@example.com", false]]);
+
+  // The same account again is not a second copy
+  await b.signIn("first@example.com", "first-password", true);
+  accounts = await b.accounts();
+  assert.equal(accounts.length, 2);
+  assert.equal(accounts[0]!.username, "first@example.com", "it came to the front instead");
+
+  // Switch back
+  const second = accounts.find((a) => !a.front)!;
+  assert.equal((await b.call(`/api/auth/accounts/${second.id}/front`, { method: "POST" })).status, 200);
+  assert.equal((await b.accounts())[0]!.username, "second@example.com");
+
+  // Signing out ends only the one in front; the other comes forward
+  const out = await b.call("/api/auth/logout", { method: "POST" });
+  assert.equal(out.body.next, true);
+  assert.deepEqual((await b.accounts()).map((a) => a.username), ["first@example.com"]);
+
+  // Sign out of all
+  await b.signIn("second@example.com", "second-password", true);
+  assert.equal((await b.accounts()).length, 2);
+  await b.call("/api/auth/logout-all", { method: "POST" });
+  assert.equal(b.jar.has(FRONT), false);
+  assert.equal(b.jar.has(MORE), false);
+  assert.equal((await b.call("/api/auth/accounts")).status, 401);
+});
+
+test("an account in front can't switch to one it doesn't hold", async () => {
+  const b = new Browser();
+  await b.signIn("first@example.com", "first-password");
+  assert.equal((await b.call("/api/auth/accounts/not-a-session/front", { method: "POST" })).status, 404);
+});
+
+test("an organization that doesn't allow it keeps adding off", async () => {
+  const b = new Browser();
+  await b.signIn("first@example.com", "first-password");
+  process.env.MOCK_NO_ADD_ACCOUNTS = "1";
+  try {
+    // The cached upstream session is a minute old at most; ask afresh
+    await b.call("/api/auth/session?refresh=1");
+    const res = await b.call("/api/auth/accounts");
+    assert.equal(res.body.canAdd, false);
+    const added = await b.signIn("second@example.com", "second-password", true);
+    assert.equal(added.status, 403);
+    assert.equal(added.body.error, "add_not_allowed");
+    assert.deepEqual((await b.accounts()).map((a) => a.username), ["first@example.com"], "the front stayed");
+  } finally {
+    delete process.env.MOCK_NO_ADD_ACCOUNTS;
+  }
+});
diff --git a/server/src/accounts.ts b/server/src/accounts.ts
new file mode 100644
index 0000000..820a8ae
--- /dev/null
+++ b/server/src/accounts.ts
@@ -0,0 +1,58 @@
+/**
+ * More than one signed-in account in a browser (multi-account spec, MA-B).
+ *
+ * The session cookie is unchanged: it is the account in front, and every
+ * request is answered with it, so nothing else in the server has to know
+ * there may be others. The others ride in a second cookie, `<name>_more`: a
+ * list of their own session cookies, each `id.secret` exactly as the front one
+ * is. Switching swaps one of them into front; adding moves the front one into
+ * the list. Each session stays its own -- its own sealed credential, its own
+ * expiry, its own "this is my device" -- and nothing about one can be read
+ * through another.
+ *
+ * At most `MAX_ACCOUNTS` in all, all on the same mail server (MA-9), and only
+ * while both accounts' organizations allow it (`addAccounts`, MA-C).
+ */
+import type { UpstreamSession } from "./upstream.js";
+
+export const MAX_ACCOUNTS = 5;
+
+/** A session cookie's shape: `id.secret`, both base64url. Anything else is dropped. */
+const COOKIE_SHAPE = /^[A-Za-z0-9_-]{8,128}\.[A-Za-z0-9_-]{8,256}$/;
+const SEP = "~";
+
+export function parseOthers(value: string | undefined): string[] {
+  if (!value) return [];
+  const seen = new Set<string>();
+  const out: string[] = [];
+  for (const part of value.split(SEP)) {
+    if (!COOKIE_SHAPE.test(part) || seen.has(part)) continue;
+    seen.add(part);
+    out.push(part);
+    if (out.length >= MAX_ACCOUNTS - 1) break;
+  }
+  return out;
+}
+
+export function serializeOthers(cookies: string[]): string {
+  return cookies.filter((c) => COOKIE_SHAPE.test(c)).slice(0, MAX_ACCOUNTS - 1).join(SEP);
+}
+
+/**
+ * Whether the account behind `upstream` may have other accounts beside it:
+ * `addAccounts` on its own account's `urn:inbuxa:jmap` capability. A server
+ * that doesn't say (an older one, or not inbuxa) allows it, as before.
+ */
+export function mayAddAccounts(upstream: UpstreamSession): boolean {
+  const primary = upstream.primaryAccounts?.["urn:ietf:params:jmap:mail"] ?? Object.keys(upstream.accounts ?? {})[0];
+  const account = primary ? (upstream.accounts?.[primary] as { accountCapabilities?: Record<string, unknown> } | undefined) : undefined;
+  const inbuxa = account?.accountCapabilities?.["urn:inbuxa:jmap"] as { addAccounts?: unknown } | undefined;
+  return inbuxa?.addAccounts !== false;
+}
+
+/** Why an account can't be added, in words for the person. */
+export const ADD_REFUSED: Record<string, string> = {
+  add_full: `You can have at most ${MAX_ACCOUNTS} accounts open here.`,
+  add_not_allowed: "Your organization doesn't allow adding other accounts here.",
+  add_other_server: "That account is on another mail server. Only accounts on this server can be added.",
+};
diff --git a/server/src/app.ts b/server/src/app.ts
index c350bab..86fec4b 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -45,6 +45,7 @@ import { SignInError, finish as finishSignIn, needsRefresh, oauthEnabled, passwo
 import { icsProxyHandler } from "./icsproxy.js";
 import { staticHandler } from "./static.js";
 import { mailNode, webmailNode } from "./nodes.js";
+import { ADD_REFUSED, MAX_ACCOUNTS, mayAddAccounts, parseOthers, serializeOthers } from "./accounts.js";
 
 type Env = { Variables: { session: LiveSession } };
 
@@ -330,6 +331,93 @@ function setSessionCookie(c: Context, value: string, remember: boolean) {
   });
 }
 
+/*
+ * inbuxa MA-B: the other signed-in accounts, beside the one in front. See
+ * accounts.ts. Kept across a browser restart only when every account in it
+ * would be (MA-7).
+ */
+const OTHERS_COOKIE = `${config.cookieName}_more`;
+
+function setOthersCookie(c: Context, cookies: string[]) {
+  if (!cookies.length) {
+    deleteCookie(c, OTHERS_COOKIE, { path: cookiePath });
+    return;
+  }
+  const remember = cookies.every((cookie) => sessions.resolve(cookie)?.remember === true);
+  setCookie(c, OTHERS_COOKIE, serializeOthers(cookies), {
+    httpOnly: true,
+    sameSite: "Lax",
+    secure: isSecureRequest(c),
+    path: cookiePath,
+    ...(remember ? { maxAge: config.sessionRememberTtl } : {}),
+  });
+}
+
+/** The other accounts still signed in, in their order; ended ones are left out. */
+function liveOthers(c: Context): { cookie: string; session: LiveSession }[] {
+  const out: { cookie: string; session: LiveSession }[] = [];
+  for (const cookie of parseOthers(getCookie(c, OTHERS_COOKIE))) {
+    const session = sessions.resolve(cookie);
+    if (session) out.push({ cookie, session });
+  }
+  return out;
+}
+
+/** Whether the account in front may have more beside it, or why not. */
+async function addRefusal(c: Context, front: LiveSession): Promise<string | null> {
+  if (1 + liveOthers(c).length >= MAX_ACCOUNTS) return "add_full";
+  try {
+    const upstream = await getUpstreamSession(front.id, front.authorization, upstreamFor(front.username));
+    if (!mayAddAccounts(upstream)) return "add_not_allowed";
+  } catch {
+    return "add_not_allowed";
+  }
+  return null;
+}
+
+/**
+ * A session just signed in to be added beside the one in front (MA-B). It
+ * comes to the front and the old front joins the others; or, refused, it is
+ * ended and the front stays. Returns the refusal, or null.
+ */
+async function joinAccount(
+  c: Context,
+  created: { cookie: string; session: LiveSession },
+  upstream: Awaited<ReturnType<typeof fetchUpstreamSession>>,
+): Promise<string | null> {
+  const frontCookie = getCookie(c, config.cookieName);
+  const front = sessions.resolve(frontCookie);
+  if (!front || !frontCookie) {
+    // Nobody in front any more: an ordinary sign-in
+    setSessionCookie(c, created.cookie, created.session.remember);
+    return null;
+  }
+  const others = liveOthers(c);
+  const end = (code: string | null) => {
+    sessions.destroy(created.session.id);
+    forgetUpstreamSession(created.session.id);
+    return code;
+  };
+  if (upstreamFor(created.session.username) !== upstreamFor(front.username)) return end("add_other_server");
+  // Both organizations must allow it
+  if (!mayAddAccounts(upstream)) return end("add_not_allowed");
+  const frontRefusal = await addRefusal(c, front);
+  if (frontRefusal === "add_not_allowed") return end(frontRefusal);
+  // Already open: that one comes to the front instead of a second copy
+  if (created.session.account === front.account) return end(null);
+  const existing = others.find((o) => o.session.account === created.session.account);
+  if (existing) {
+    end(null);
+    setSessionCookie(c, existing.cookie, existing.session.remember);
+    setOthersCookie(c, [frontCookie, ...others.filter((o) => o !== existing).map((o) => o.cookie)]);
+    return null;
+  }
+  if (1 + others.length >= MAX_ACCOUNTS) return end("add_full");
+  setSessionCookie(c, created.cookie, created.session.remember);
+  setOthersCookie(c, [frontCookie, ...others.map((o) => o.cookie)]);
+  return null;
+}
+
 function upstreamFailure(c: Context, err: unknown) {
   if (err instanceof UpstreamError) {
     return c.json({ error: err.status === 401 ? "invalid_credentials" : "upstream_error", message: err.message }, err.status as 401 | 502);
@@ -406,8 +494,19 @@ export function createApp(basePath = config.basePath): Hono<Env> {
       return c.redirect(`${basePath}/?signin_error=rate_limited`, 302);
     }
     const username = (c.req.query("username") ?? "").trim().slice(0, 320);
+    // inbuxa MA-B: another account beside the one in front, if it may have one
+    const front = c.req.query("add") === "1" ? sessions.resolve(getCookie(c, config.cookieName)) : null;
+    if (front) {
+      const refused = await addRefusal(c, front);
+      if (refused) return c.redirect(`${basePath}/?account_error=${refused}`, 302);
+    }
     try {
-      const { location, state } = await startSignIn({ username, base: upstreamFor(username), remember: c.req.query("remember") === "1" });
+      const { location, state } = await startSignIn({
+        username,
+        base: upstreamFor(username),
+        remember: c.req.query("remember") === "1",
+        adding: front !== null,
+      });
       setCookie(c, OAUTH_STATE_COOKIE, state, { httpOnly: true, sameSite: "Lax", secure: isSecureRequest(c), path: `${basePath}/api/auth`, maxAge: 600 });
       return c.redirect(location, 302);
     } catch (err) {
@@ -452,6 +551,11 @@ export function createApp(basePath = config.basePath): Hono<Env> {
         userAgent: c.req.header("user-agent") ?? "",
         ip: clientIp(c),
       });
+      if (result.adding) {
+        const refused = await joinAccount(c, { cookie, session }, upstream);
+        if (refused) return c.redirect(`${basePath}/?account_error=${refused}`, 302);
+        return c.redirect(`${basePath}/`, 302);
+      }
       setSessionCookie(c, cookie, session.remember);
       const mailAccount = upstream.primaryAccounts?.["urn:ietf:params:jmap:mail"];
       if (mailAccount) pushPrepare(session.username, mailAccount, pushCredential(session));
@@ -473,7 +577,7 @@ export function createApp(basePath = config.basePath): Hono<Env> {
       c.header("Retry-After", String(loginFloodLimiter.retryAfterSeconds(rateIp)));
       return c.json({ error: "rate_limited", message: "Too many login attempts. Please wait and try again." }, 429);
     }
-    let body: { username?: string; password?: string; totp?: string; remember?: boolean };
+    let body: { username?: string; password?: string; totp?: string; remember?: boolean; add?: boolean };
     try {
       body = await c.req.json();
     } catch {
@@ -536,6 +640,12 @@ export function createApp(basePath = config.basePath): Hono<Env> {
         userAgent: c.req.header("user-agent") ?? "",
         ip,
       });
+      // inbuxa MA-B: beside the account in front, when that's what was asked
+      if (body.add && sessions.resolve(getCookie(c, config.cookieName))) {
+        const refused = await joinAccount(c, { cookie, session }, upstream);
+        if (refused) return c.json({ error: refused, message: ADD_REFUSED[refused] }, 403);
+        return c.json({ ok: true, added: true });
+      }
       setSessionCookie(c, cookie, session.remember);
       // Start the account's push subscription now, so it is usually verified
       // by the time the browser opens its stream. See push.ts.
@@ -606,7 +716,54 @@ export function createApp(basePath = config.basePath): Hono<Env> {
       sessions.destroy(session.id);
       forgetUpstreamSession(session.id);
     }
+    // inbuxa MA-B: only this account ends; the next one comes to the front
+    const [next, ...rest] = liveOthers(c);
+    if (next) {
+      setSessionCookie(c, next.cookie, next.session.remember);
+      setOthersCookie(c, rest.map((o) => o.cookie));
+      return c.json({ ok: true, next: true });
+    }
     deleteCookie(c, config.cookieName, { path: cookiePath });
+    deleteCookie(c, OTHERS_COOKIE, { path: cookiePath });
+    return c.json({ ok: true });
+  });
+
+  /* inbuxa MA-B: every account signed in here ends. */
+  api.post("/auth/logout-all", async (c) => {
+    const front = sessions.resolve(getCookie(c, config.cookieName));
+    for (const session of [front, ...liveOthers(c).map((o) => o.session)]) {
+      if (!session) continue;
+      sessions.destroy(session.id);
+      forgetUpstreamSession(session.id);
+    }
+    deleteCookie(c, config.cookieName, { path: cookiePath });
+    deleteCookie(c, OTHERS_COOKIE, { path: cookiePath });
+    return c.json({ ok: true });
+  });
+
+  /* inbuxa MA-B: the accounts signed in here, the one in front first, and whether one more may be added. */
+  api.get("/auth/accounts", requireSession, async (c) => {
+    const front = c.get("session");
+    const others = liveOthers(c);
+    if (others.length !== parseOthers(getCookie(c, OTHERS_COOKIE)).length) {
+      setOthersCookie(c, others.map((o) => o.cookie));
+    }
+    const canAdd = (await addRefusal(c, front)) === null;
+    return c.json({
+      accounts: [front, ...others.map((o) => o.session)].map((s, i) => ({ id: s.id, username: s.username, front: i === 0 })),
+      canAdd,
+      max: MAX_ACCOUNTS,
+    });
+  });
+
+  /* inbuxa MA-B: bring another signed-in account to the front. */
+  api.post("/auth/accounts/:id/front", requireSession, async (c) => {
+    const frontCookie = getCookie(c, config.cookieName)!;
+    const others = liveOthers(c);
+    const chosen = others.find((o) => o.session.id === c.req.param("id"));
+    if (!chosen) return c.json({ error: "not_found" }, 404);
+    setSessionCookie(c, chosen.cookie, chosen.session.remember);
+    setOthersCookie(c, [frontCookie, ...others.filter((o) => o !== chosen).map((o) => o.cookie)]);
     return c.json({ ok: true });
   });
 
diff --git a/server/src/mock/index.ts b/server/src/mock/index.ts
index 491e2de..748ae31 100644
--- a/server/src/mock/index.ts
+++ b/server/src/mock/index.ts
@@ -23,9 +23,25 @@ function unauthorized(res: ServerResponse) {
   res.end(JSON.stringify({ type: "about:blank", status: 401, title: "Unauthorized" }));
 }
 
+/*
+ * inbuxa MA-B: an optional second user, so the account switcher has two real
+ * accounts to move between. It signs in with a password only, and reads the
+ * same mailbox: what the tests look at is who the session says it is.
+ */
+const SECOND_USER = process.env.MOCK_SECOND_USER;
+const SECOND_PASS = process.env.MOCK_SECOND_PASS;
+
+/** Who a Basic header signs in as, when it is the second user. */
+function secondUser(req: IncomingMessage): boolean {
+  const h = req.headers.authorization ?? "";
+  if (!SECOND_USER || !h.startsWith("Basic ")) return false;
+  return Buffer.from(h.slice(6), "base64").toString() === `${SECOND_USER}:${SECOND_PASS}`;
+}
+
 function checkAuth(req: IncomingMessage): boolean {
   const h = req.headers.authorization ?? "";
   if (checkBearer(h)) return true;
+  if (secondUser(req)) return true;
   if (!h.startsWith("Basic ") || basicRefused) return false;
   const raw = Buffer.from(h.slice(6), "base64").toString();
   const sep = raw.indexOf(":");
@@ -83,7 +99,14 @@ export const server = createServer(async (req, res) => {
   if (!checkAuth(req)) return unauthorized(res);
   if (url.pathname === "/.well-known/jmap" || url.pathname === "/jmap/session") {
     res.writeHead(200, { "content-type": "application/json" });
-    return res.end(JSON.stringify(session()));
+    const answer = session() as ReturnType<typeof session> & { username: string };
+    if (secondUser(req)) answer.username = SECOND_USER!;
+    // MA-C: an organization that doesn't allow adding accounts
+    if (process.env.MOCK_NO_ADD_ACCOUNTS === "1") {
+      const own = answer.accounts[ACCOUNT] as { accountCapabilities: Record<string, unknown> };
+      own.accountCapabilities = { ...own.accountCapabilities, "urn:inbuxa:jmap": { addAccounts: false } };
+    }
+    return res.end(JSON.stringify(answer));
   }
   // The account info endpoint; the only place a server reports its edition.
   if (url.pathname === "/api/account" && req.method === "GET") {
diff --git a/server/src/oauth.test.ts b/server/src/oauth.test.ts
index 236111c..e15528b 100644
--- a/server/src/oauth.test.ts
+++ b/server/src/oauth.test.ts
@@ -217,3 +217,23 @@ test("push keeps a credential that renews itself", async () => {
   assert.notEqual(second, first, "a fresh access token");
   assert.match(second, /^Bearer mock-at-/);
 });
+
+test("inbuxa MA-B: adding an account asks the server's page to sign in again", async () => {
+  // With nobody in front, add=1 is an ordinary sign-in
+  let res = await call("/api/auth/oauth/start?username=demo@example.com&add=1");
+  assert.equal(new URL(res.headers.get("location")!).searchParams.has("prompt"), false);
+
+  await signIn();
+  res = await call("/api/auth/oauth/start?username=demo@example.com&add=1");
+  assert.equal(res.status, 302);
+  const signInPage = new URL(res.headers.get("location")!);
+  assert.equal(signInPage.searchParams.get("prompt"), "login", "the server's page must not reuse the first sign-in");
+
+  // The mock's page signs the same account in again: it stays one account
+  const approved = await fetch(signInPage, { redirect: "manual" });
+  const back = new URL(approved.headers.get("location")!);
+  res = await call(`/api/auth/callback${back.search}`);
+  assert.equal(res.headers.get("location"), "/");
+  const list = await jsonOf(await call("/api/auth/accounts"));
+  assert.deepEqual(list.accounts.map((a: { username: string }) => a.username), ["demo@example.com"]);
+});
diff --git a/server/src/oauth.ts b/server/src/oauth.ts
index e9c81d0..9cd7598 100644
--- a/server/src/oauth.ts
+++ b/server/src/oauth.ts
@@ -97,6 +97,8 @@ interface Pending {
   base: string;
   username: string;
   remember: boolean;
+  /** MA-B: signing in a second account beside the one in front. */
+  adding: boolean;
   createdAt: number;
 }
 
@@ -114,13 +116,13 @@ function challengeOf(verifier: string): string {
  * Begin a sign-in. Returns where to send the browser, and the state to bind
  * to it in a cookie.
  */
-export async function start(params: { username: string; base: string; remember: boolean }): Promise<{ location: string; state: string }> {
+export async function start(params: { username: string; base: string; remember: boolean; adding?: boolean }): Promise<{ location: string; state: string }> {
   const metadata = await metadataFor(params.base);
   sweepPending();
   if (pending.size >= MAX_PENDING) throw new UpstreamError("Too many sign-ins in progress", 503);
   const state = randomToken(24);
   const verifier = randomToken(48);
-  pending.set(state, { verifier, base: params.base, username: params.username, remember: params.remember, createdAt: Date.now() });
+  pending.set(state, { verifier, base: params.base, username: params.username, remember: params.remember, adding: Boolean(params.adding), createdAt: Date.now() });
   const scope = ["openid", "offline_access"].filter((s) => metadata.scopes.length === 0 || metadata.scopes.includes(s)).join(" ");
   const url = new URL(metadata.authorizationEndpoint);
   url.searchParams.set("response_type", "code");
@@ -131,6 +133,9 @@ export async function start(params: { username: string; base: string; remember:
   url.searchParams.set("code_challenge", challengeOf(verifier));
   url.searchParams.set("code_challenge_method", "S256");
   if (params.username) url.searchParams.set("login_hint", params.username);
+  // MA-B: ask again, rather than let the server's page reuse the sign-in of
+  // the account already in front
+  if (params.adding) url.searchParams.set("prompt", "login");
   return { location: url.toString(), state };
 }
 
@@ -177,7 +182,7 @@ export class SignInError extends Error {
  * Finish a sign-in: `state` as it came back in the URL, `boundState` as the
  * browser's cookie holds it. Each state is good for one attempt.
  */
-export async function finish(params: { state: string; boundState: string | undefined; code: string }): Promise<{ tokens: TokenSet; base: string; username: string; remember: boolean }> {
+export async function finish(params: { state: string; boundState: string | undefined; code: string }): Promise<{ tokens: TokenSet; base: string; username: string; remember: boolean; adding: boolean }> {
   const p = pending.get(params.state);
   if (!p || !params.boundState || params.boundState !== params.state) {
     throw new SignInError("state_mismatch", "This sign-in didn't start in this browser. Try again.");
@@ -192,7 +197,7 @@ export async function finish(params: { state: string; boundState: string | undef
     redirect_uri: redirectUri(),
   });
   if (!tokens) throw new SignInError("exchange_failed", "The mail server didn't accept the sign-in. Try again.");
-  return { tokens, base: p.base, username: p.username, remember: p.remember };
+  return { tokens, base: p.base, username: p.username, remember: p.remember, adding: p.adding };
 }
 
 /**
diff --git a/web/src/store/__tests__/account-switcher.test.ts b/web/src/store/__tests__/account-switcher.test.ts
new file mode 100644
index 0000000..b5022f5
--- /dev/null
+++ b/web/src/store/__tests__/account-switcher.test.ts
@@ -0,0 +1,77 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { useSession } from "@/store/session";
+
+/**
+ * inbuxa MA-B: the accounts signed in in this browser. The server lists them
+ * and swaps the one in front; the web app clears what it cached for the
+ * previous account and reloads, and signing out of one reloads into the next.
+ */
+
+let reload: ReturnType<typeof vi.fn>;
+let calls: { url: string; method: string }[];
+let answers: Record<string, unknown>;
+
+beforeEach(() => {
+  reload = vi.fn();
+  Object.defineProperty(window, "location", { configurable: true, value: { ...window.location, reload } });
+  calls = [];
+  answers = {};
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async (url: string, init?: RequestInit) => {
+      const path = String(url);
+      calls.push({ url: path, method: init?.method ?? "GET" });
+      const key = Object.keys(answers).find((k) => path.endsWith(k));
+      const body = key ? answers[key] : { ok: true };
+      return { ok: true, status: 200, json: async () => body, text: async () => JSON.stringify(body) } as Response;
+    }),
+  );
+  localStorage.setItem("ihasmail:cached-thing", "from the account in front");
+  useSession.setState({ status: "authenticated", signedIn: [], canAddAccount: false });
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  localStorage.clear();
+});
+
+describe("account switcher", () => {
+  it("lists the accounts and whether one more may be added", async () => {
+    answers["/api/auth/accounts"] = {
+      accounts: [
+        { id: "a", username: "first@example.com", front: true },
+        { id: "b", username: "second@example.com", front: false },
+      ],
+      canAdd: true,
+    };
+    await useSession.getState().loadSignedIn();
+    expect(useSession.getState().signedIn.map((a) => a.username)).toEqual(["first@example.com", "second@example.com"]);
+    expect(useSession.getState().canAddAccount).toBe(true);
+  });
+
+  it("switching asks the server, forgets the cache and reloads", async () => {
+    await useSession.getState().switchTo("b");
+    expect(calls.some((c) => c.url.endsWith("/api/auth/accounts/b/front") && c.method === "POST")).toBe(true);
+    expect(localStorage.getItem("ihasmail:cached-thing")).toBeNull();
+    expect(reload).toHaveBeenCalledOnce();
+  });
+
+  it("signing out of one reloads into the next when there is one", async () => {
+    answers["/api/auth/logout"] = { ok: true, next: true };
+    await useSession.getState().logout();
+    expect(reload).toHaveBeenCalledOnce();
+  });
+
+  it("signing out of the last one, or of all, ends at the sign-in page", async () => {
+    answers["/api/auth/logout-all"] = { ok: true };
+    await useSession.getState().logoutAll();
+    expect(calls.some((c) => c.url.endsWith("/api/auth/logout-all"))).toBe(true);
+    expect(reload).not.toHaveBeenCalled();
+    expect(useSession.getState().status).toBe("anonymous");
+    // The next ordinary sign-out goes back to signing out of one
+    useSession.setState({ status: "authenticated" });
+    answers["/api/auth/logout"] = { ok: true };
+    await useSession.getState().logout();
+    expect(calls.filter((c) => c.url.endsWith("/api/auth/logout")).length).toBe(1);
+  });
+});
diff --git a/web/src/store/session.ts b/web/src/store/session.ts
index 1a9582f..9f46b14 100644
--- a/web/src/store/session.ts
+++ b/web/src/store/session.ts
@@ -28,6 +28,10 @@ interface SessionState {
   viewing: Id | null;
   /** Shared and group mailboxes the reader can open (MA-A); see lib/sharedMail. */
   sharedMail: SharedMailAccount[];
+  /** inbuxa MA-B: the accounts signed in in this browser, the one in front first. */
+  signedIn: SignedInAccount[];
+  /** Whether one more may be added (the cap, and both organizations' addAccounts). */
+  canAddAccount: boolean;
   /** The name of an account the reader lost while it was in view. */
   delegationEnded: string | null;
   error: string | null;
@@ -36,7 +40,14 @@ interface SessionState {
   pushState: PushState;
   bootstrap(): Promise<void>;
   login(username: string, password: string, totp: string, remember: boolean): Promise<void>;
+  /** Signs out of the account in front; another signed-in one comes forward. */
   logout(): Promise<void>;
+  /** inbuxa MA-B: ends every account signed in in this browser. */
+  logoutAll(): Promise<void>;
+  /** inbuxa MA-B: finds the other accounts signed in here. */
+  loadSignedIn(): Promise<void>;
+  /** inbuxa MA-B: brings another signed-in account to the front, and reloads. */
+  switchTo(sessionId: string): Promise<void>;
   refresh(): Promise<void>;
   setAccount(id: Id): void;
   /** Show a delegated account's or shared mailbox's mail, or the reader's own with null. */
@@ -59,12 +70,24 @@ interface SessionState {
 
 let refreshing: Promise<void> | null = null;
 
+/** What a signed-in account looks like in the switcher (MA-B). */
+export interface SignedInAccount {
+  id: string;
+  username: string;
+  front: boolean;
+}
+
+/** Which sign-out: the account in front, or every one (MA-B). */
+let signOutPath = "/api/auth/logout";
+
 export const useSession = create<SessionState>((set, get) => ({
   status: "loading",
   session: null,
   accountId: null,
   viewing: null,
   sharedMail: [],
+  signedIn: [],
+  canAddAccount: false,
   delegationEnded: null,
   error: null,
   pushConnected: false,
@@ -116,8 +139,11 @@ export const useSession = create<SessionState>((set, get) => ({
     } catch {
       /* never block signing out over this */
     }
+    const path = signOutPath;
+    signOutPath = "/api/auth/logout";
+    let next = false;
     try {
-      await apiFetch("/api/auth/logout", { method: "POST" });
+      next = Boolean((await apiFetch<{ next?: boolean }>(path, { method: "POST" }))?.next);
     } catch {
       /* ignore */
     }
@@ -127,7 +153,34 @@ export const useSession = create<SessionState>((set, get) => ({
     // problem next -- and the address book cached here is the same argument.
     clearSignedInData();
     client.session = null;
-    set({ status: "anonymous", session: null, accountId: null, viewing: null, sharedMail: [] });
+    // inbuxa MA-B: another signed-in account is in front now
+    if (next) {
+      window.location.reload();
+      return;
+    }
+    set({ status: "anonymous", session: null, accountId: null, viewing: null, sharedMail: [], signedIn: [], canAddAccount: false });
+  },
+
+  async logoutAll() {
+    // The same care as signing out of one, then every account ends
+    signOutPath = "/api/auth/logout-all";
+    await get().logout();
+  },
+
+  async loadSignedIn() {
+    try {
+      const answer = await apiFetch<{ accounts: SignedInAccount[]; canAdd: boolean }>("/api/auth/accounts");
+      set({ signedIn: answer.accounts, canAddAccount: answer.canAdd });
+    } catch {
+      set({ signedIn: [], canAddAccount: false });
+    }
+  },
+
+  async switchTo(sessionId) {
+    await apiFetch(`/api/auth/accounts/${encodeURIComponent(sessionId)}/front`, { method: "POST" });
+    // What was cached belongs to the account that was in front
+    clearSignedInData();
+    window.location.reload();
   },
 
   refresh() {
@@ -213,6 +266,7 @@ function applySession(s: JmapSession, set: (p: Partial<SessionState>) => void) {
   const accountId = s.primaryAccounts[CAP.mail] ?? Object.keys(s.accounts)[0] ?? null;
   set({ status: "authenticated", session: s, accountId, viewing: null, sharedMail: [], error: null });
   void useSession.getState().loadSharedMail();
+  void useSession.getState().loadSignedIn();
 }
 
 client.onUnauthenticated(() => {
diff --git a/web/src/views/AddAccountDialog.tsx b/web/src/views/AddAccountDialog.tsx
new file mode 100644
index 0000000..2f76250
--- /dev/null
+++ b/web/src/views/AddAccountDialog.tsx
@@ -0,0 +1,111 @@
+import { useEffect, useState, type FormEvent } from "react";
+import { Dialog } from "@/ui/dialog";
+import { apiFetch } from "@/jmap/client";
+import { withBase } from "@/lib/basePath";
+import { clearSignedInData, isDeviceTrusted } from "@/lib/storage";
+import { t } from "@/lib/i18n";
+
+/**
+ * inbuxa MA-B: sign a second account in beside the one in front.
+ *
+ * With sign-in on the mail server's own page and one mail server, there is
+ * nothing to ask here: the browser goes straight to that page, which asks for
+ * the account. With several servers the address comes first, to pick one; with
+ * the password form, so does the password. Either way the account joins the
+ * others and comes to the front, and the app reloads into it.
+ *
+ * It is remembered on this device exactly as the first one was (MA-7).
+ */
+export function AddAccountDialog({ open, onClose }: { open: boolean; onClose: () => void }) {
+  const [mode, setMode] = useState<"oauth" | "oauth-address" | "password" | null>(null);
+  const [username, setUsername] = useState("");
+  const [password, setPassword] = useState("");
+  const [busy, setBusy] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const remember = isDeviceTrusted();
+
+  useEffect(() => {
+    if (!open) return;
+    setError(null);
+    let live = true;
+    fetch(withBase("/api/config"))
+      .then((r) => (r.ok ? r.json() : null))
+      .then((c) => {
+        if (!live) return;
+        const oauth = c?.signIn === "oauth";
+        const next = oauth ? (c?.signInDirect === true ? "oauth" : "oauth-address") : "password";
+        setMode(next);
+        // Nothing to ask: off to the mail server's page
+        if (next === "oauth") goToServer("");
+      })
+      .catch(() => live && setMode("password"));
+    return () => {
+      live = false;
+    };
+    // goToServer only reads `remember`, fixed for the dialog's life
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [open]);
+
+  function goToServer(address: string) {
+    setBusy(true);
+    // What is cached belongs to the account in front, which won't be
+    clearSignedInData();
+    const params = new URLSearchParams({ add: "1", ...(address ? { username: address } : {}), ...(remember ? { remember: "1" } : {}) });
+    window.location.assign(withBase(`/api/auth/oauth/start?${params}`));
+  }
+
+  const submit = async (e: FormEvent) => {
+    e.preventDefault();
+    if (!username.trim()) return;
+    if (mode === "oauth-address") return goToServer(username.trim());
+    if (!password) return;
+    setBusy(true);
+    setError(null);
+    try {
+      await apiFetch("/api/auth/login", {
+        method: "POST",
+        body: JSON.stringify({ username: username.trim(), password, remember, add: true }),
+      });
+      clearSignedInData();
+      window.location.reload();
+    } catch (err) {
+      setError((err as Error).message || t("That account couldn't be added."));
+      setBusy(false);
+    }
+  };
+
+  if (mode === "oauth") return null;
+  return (
+    <Dialog
+      open={open}
+      onClose={onClose}
+      title={t("Add an account")}
+      size="sm"
+      footer={
+        <>
+          <button className="btn btn-ghost" onClick={onClose} disabled={busy}>
+            {t("Cancel")}
+          </button>
+          <button className="btn btn-primary" form="add-account" type="submit" disabled={busy || !username.trim() || (mode === "password" && !password)}>
+            {busy ? t("Working…") : t("Add account")}
+          </button>
+        </>
+      }
+    >
+      <form id="add-account" onSubmit={(e) => void submit(e)}>
+        <p className="hint">{t("Both accounts stay signed in here; switch between them from this menu.")}</p>
+        <div className="field">
+          <label htmlFor="add-account-user">{t("Email address")}</label>
+          <input id="add-account-user" autoComplete="username" value={username} onChange={(e) => setUsername(e.target.value)} autoFocus />
+        </div>
+        {mode === "password" && (
+          <div className="field">
+            <label htmlFor="add-account-pw">{t("Password")}</label>
+            <input id="add-account-pw" type="password" autoComplete="current-password" value={password} onChange={(e) => setPassword(e.target.value)} />
+          </div>
+        )}
+        {error && <p className="error">{error}</p>}
+      </form>
+    </Dialog>
+  );
+}
diff --git a/web/src/views/AppShell.tsx b/web/src/views/AppShell.tsx
index a0a28a5..b032e04 100644
--- a/web/src/views/AppShell.tsx
+++ b/web/src/views/AppShell.tsx
@@ -11,6 +11,7 @@ import { composeBlocked, draftFromMailto, useCompose } from "@/store/compose";
 import { delegatedAccounts } from "@/lib/delegation";
 import { toast } from "@/ui/toast";
 import { DelegatedBar } from "./DelegatedBar";
+import { AddAccountDialog } from "./AddAccountDialog";
 import { Avatar, useIsMobile } from "@/ui/misc";
 import { MenuItem, MenuSep, Popover, useMenu } from "@/ui/popover";
 import { Splitter } from "@/ui/Splitter";
@@ -73,6 +74,35 @@ export function AppShell({ children }: { children: ReactNode }) {
   const pushState = useSession((s) => s.pushState);
   const session = useSession((s) => s.session);
   const logout = useSession((s) => s.logout);
+  // inbuxa MA-B: the other accounts signed in here, and adding one
+  const signedIn = useSession((s) => s.signedIn);
+  const canAddAccount = useSession((s) => s.canAddAccount);
+  const [addingAccount, setAddingAccount] = useState(false);
+  const bringForward = (sessionId: string) => {
+    acctMenu.close();
+    // A message being written belongs to the account it was started in
+    if (useCompose.getState().drafts.length) {
+      toast.show(t("Send or close the message you're writing first."));
+      return;
+    }
+    void useSession.getState().switchTo(sessionId);
+  };
+  // A refused add comes back on the address (see the server's /auth/callback)
+  useEffect(() => {
+    const params = new URLSearchParams(window.location.search);
+    const why = params.get("account_error");
+    if (!why) return;
+    toast.show(
+      why === "add_full"
+        ? t("You can't add more accounts here.")
+        : why === "add_other_server"
+          ? t("That account is on another mail server. Only accounts on this server can be added.")
+          : t("Your organization doesn't allow adding other accounts here."),
+    );
+    params.delete("account_error");
+    const rest = params.toString();
+    window.history.replaceState(null, "", window.location.pathname + (rest ? `?${rest}` : ""));
+  }, []);
   const appName = useSession((s) => s.session?.ihasmail?.appName) || DEFAULT_APP_NAME;
   const acctMenu = useMenu();
   const administers = hasAdministration(usePermissions());
@@ -220,6 +250,33 @@ export function AppShell({ children }: { children: ReactNode }) {
               </div>
             </div>
             <MenuSep />
+            {/* inbuxa MA-B: each account signed in here, the one in front ticked */}
+            {(signedIn.length > 1 || canAddAccount) && (
+              <>
+                {signedIn.length > 1 && <div className="hint" style={{ padding: "4px 10px" }}>{t("Accounts")}</div>}
+                {signedIn.length > 1 &&
+                  signedIn.map((account) => (
+                    <MenuItem
+                      key={account.id}
+                      icon={account.front ? <Check size={16} /> : <Mail size={16} />}
+                      label={<span className="notranslate" translate="no">{account.username}</span>}
+                      active={account.front}
+                      onClick={() => (account.front ? acctMenu.close() : bringForward(account.id))}
+                    />
+                  ))}
+                {canAddAccount && (
+                  <MenuItem
+                    icon={<Plus size={16} />}
+                    label={t("Add account")}
+                    onClick={() => {
+                      acctMenu.close();
+                      setAddingAccount(true);
+                    }}
+                  />
+                )}
+                <MenuSep />
+              </>
+            )}
             {delegated.length + sharedMail.length > 0 && (
               <>
                 <div className="hint" style={{ padding: "4px 10px" }}>{t("Mail to show")}</div>
@@ -277,7 +334,11 @@ export function AppShell({ children }: { children: ReactNode }) {
             )}
             <MenuItem icon={<RefreshCw size={16} />} label={t("Refresh")} onClick={() => window.location.reload()} />
             <MenuItem icon={<LogOut size={16} />} label={t("Sign out")} onClick={() => void logout()} />
+            {signedIn.length > 1 && (
+              <MenuItem icon={<LogOut size={16} />} label={t("Sign out of all accounts")} onClick={() => void useSession.getState().logoutAll()} />
+            )}
           </Popover>
+          <AddAccountDialog open={addingAccount} onClose={() => setAddingAccount(false)} />
         </div>
       </header>