inbuxa-server/features/masked-email.md

Feature spec: masked email

Status: draft, 2026-09-18. Feature 2 in SPEC.md §4.

Provenance

Written for the clean room (SPEC.md §3). Sources, and nothing else:

Source	License	Used for
Stalwart's registry schema, x:MaskedEmail and x:Email.maxMaskedAddresses, upstream d9dee0a	AGPL-3.0-only OR LicenseRef-SEL, taken under the AGPL	The stored record, field meanings, permissions
Stalwart documentation: "Masked email" (email/management/masked-email.md) and the MaskedEmail object reference	Unlicensed public documentation: facts used, prose not copied	Lifecycle, domain choice, quota, permissions, API
Fastmail's Masked Email API (https://www.fastmail.com/for-developers/masked-email/)	Published vendor API: facts used, prose not copied	The second API, its states and rules
RFC 8620	IETF	/get and /set semantics, SetError types
Observation of INBUXA's live server	Observation	Everything under "To observe" once settled

No Enterprise-only file or snippet was used. As with multi-tenancy, the drafting session writes specs only, and gaps are settled by observation or marked Decision. None is filled from memory of upstream code.

What it is

A masked address is a disposable address that delivers to one account without revealing the account's real address. A user hands a different one to each service, and can cut any of them off without touching the others. Password managers can create them automatically when a new login is saved.

Upstream ships this only in its Enterprise Edition. inbuxa-server ships it to everybody, and serves it through two APIs at once: upstream's, so existing clients keep working, and Fastmail's published one, so password managers that already speak it work without changes.

The key fact for compatibility

Every stored mask carries its full address in email. Delivery therefore finds a mask by looking the recipient address up among stored masks, and never by working out who owns an address from its shape. That is how every address INBUXA users already hold keeps delivering: inbuxa-server doesn't need to know how upstream built them.

Data model

Upstream's record, x:MaskedEmail, unchanged

Field	Upstream mutability	Meaning
accountId	immutable	The owning account
email	server-set	The address
enabled	mutable, default true	Upstream: whether mail is accepted
description	mutable	A short note from the user
forDomain	mutable	The origin of the site it was made for, e.g. https://example.com
url	mutable	A deep link back into the integrator's own record
createdAt	server-set	When it was made
createdBy	mutable	The creating client's name
expiresAt	immutable (create-only)	When it stops working. Upstream's docs say its generated addresses carry the expiry in the address itself
emailPrefix	create-only	Requested start of the local part: ≤ 64 chars, a-z, 0-9, _
emailDomain	create-only	Requested domain

Permissions: sysMaskedEmailGet, sysMaskedEmailQuery, sysMaskedEmailCreate, sysMaskedEmailUpdate, sysMaskedEmailDestroy. Limit: maxMaskedAddresses, as the server default on x:Email and per account in quotas. Unset means unlimited, and 0 turns the feature off for that account.

Added by inbuxa-server, in its own store

Kept beside the upstream record, keyed by mask id, so the upstream record stays byte-for-byte what upstream wrote:

• state: pending, enabled, disabled or deleted (see the next section). Missing for masks created before the fork: derived from enabled.
• lastMessageAt: when mail last arrived through it, or null.
• pendingUntil: for pending masks, when they're removed if no mail arrives.
• Tombstones: every address ever issued, including destroyed ones, so an address is never issued twice.

One state, two APIs

Fastmail and upstream disagree about what "disabled" means. inbuxa-server keeps one state per mask, and each API shows it in its own terms.

inbuxa-server state	Mail to it	Fastmail state	Upstream enabled
pending	Delivered, and the mask becomes enabled	pending	true
enabled	Delivered normally	enabled	true
disabled	Accepted, filed straight to Trash	disabled	true
deleted	Refused	deleted	false
(destroyed)	Refused, as an unknown address	not returned	not returned
(expired)	Refused	deleted	false

Writes map back:

• ME-1. Fastmail state sets the state directly. pending can't be set once the mask has left it, per Fastmail's rules.
• ME-2. Upstream enabled: false sets deleted. Upstream enabled: true sets enabled, from any state.
• ME-3. Upstream destroy, and Fastmail destroy, remove the mask. The address is kept as a tombstone and mail to it is refused like mail to any unknown address.

Decision: upstream's enabled: false means "reject" in its docs, which is Fastmail's deleted, not its disabled. So disabled (to Trash) has no upstream equivalent, and a mask in it reads as enabled: true there: mail is accepted, which is the fact enabled reports.

Required behavior

Delivery

• ME-4. A message to a masked address in pending, enabled or disabled is delivered to the owning account, found by stored address. Spam filtering, the account's Sieve scripts, quotas and tenant limits all apply exactly as they would to mail for the account's own address.
• ME-5. disabled delivers into the account's Trash mailbox, skipping the user's filing rules but not spam checks.
• ME-6. deleted, expired and destroyed masks refuse the message at RCPT TO. The reply code matches upstream's reply for a disabled mask (to observe, 1), so senders see the same result whichever server they meet.
• ME-7. Arriving mail sets lastMessageAt, and moves a pending mask to enabled.
• ME-8. A pending mask with no mail within 24 hours of creation is removed, per Fastmail's rule, and tombstoned. It's hidden from ihasmail's list while pending, as Fastmail's own interface hides it.
• ME-9. Delivered mail keeps the masked address visible to the user: in the To or Cc header as sent, and in a Delivered-To header naming the mask, so filters and the reader can tell which mask it came through (to observe, 3: match upstream if it already does this).
• ME-10. Sub-addressing (mask+tag@domain) on a mask works exactly as it does on the account's own addresses (to observe, 4).

Sending

• ME-11. A user may send from any of their masks in pending, enabled or disabled state, as they can from their own aliases. Replying to mail that came through a mask should default to sending from that mask: ihasmail's job (see "ihasmail"), made possible by ME-9. Whether upstream allows sending as a mask at all is to observe, 5.

Creating

• ME-12. The server generates the address. The domain is emailDomain if given, else the owning account's primary domain. emailDomain may be any domain or alias domain the account is linked to. Anything else fails with invalidProperties naming emailDomain. In a tenant, only the tenant's domains qualify (multi-tenancy MT-3).
• ME-13. Address format, Decision: the local part is {emailPrefix}_{random} when a prefix is given, else {random}, where {random} is 12 characters from a-z0-9, about 62 bits. It's checked against every address the server knows (accounts, aliases, lists, masks and tombstones) and redrawn on collision. inbuxa-server doesn't copy upstream's shape and doesn't need to. The expiry isn't encoded in the address, since it's stored in expiresAt.
• ME-14. maxMaskedAddresses counts live masks: pending, enabled, disabled. A create past it fails with overQuota. 0 turns creation off.
• ME-15. Create-rate limit per account, as Fastmail's API allows: past it, create fails with rateLimit. Decision: 50 an hour per account, configurable.
• ME-16. createdBy is set by the server from the authenticated client's name (the OAuth client's name once SPEC.md §5.2 is in place). Fastmail's API treats it as server-set. Upstream's API accepts a client-supplied value (its docs show one in a create), so the x: API still accepts it when the server has no client name of its own.
• ME-17. forDomain is stored as given. The Fastmail API asks integrators for an origin only, but inbuxa-server doesn't reject paths, since existing upstream records may hold them.

Who can do what

• ME-18. A user manages its own masks: get, query, create, update, destroy. Its role needs the sysMaskedEmail* permissions, which the default user role carries.
• ME-19. An administrator with the same permissions can manage another account's masks, for support. A tenant administrator can manage only masks owned by accounts in its tenant (multi-tenancy MT-1).

The two APIs

Upstream's, unchanged

x:MaskedEmail/get, /query, /set under urn:stalwart:jmap, standard RFC 8620 shapes, the record above, filtered by accountId in /query. Plus /changes, if upstream offers it (to observe, 6).

Fastmail's

Capability https://www.fastmail.com/dev/maskedemail, advertised in the session and in the account capabilities of every account that may hold masks.

• MaskedEmail/get (with ids: null fetching all of an account's masks) and MaskedEmail/set, standard RFC 8620 shapes, in the user's own JMAP account.
• Properties: id, email, state, forDomain, description, lastMessageAt, createdAt, createdBy, url, and emailPrefix (create-only). The same ids as the x: API, so an id is one mask whichever API reads it.
• description defaults to the empty string.

ihasmail

• A Masked addresses section in Settings: the list (address, description, site, created, last mail, state), create with an optional description and site, copy, switch between enabled, disabled (to Trash) and deleted, and destroy with a warning that the address is gone for good. Pending masks are hidden.
• Compose: a masked address can be chosen as the sender. Replying to mail that came through a mask selects that mask by default (ME-11).
• Reading: a small marker on mail that arrived through a mask, naming it.
• Administration: an account's masks on its panel, for admins with the permissions.
• Every string this adds is new translation work for ihasmail's nine languages.

Acceptance tests

1. Create with no arguments: gets a unique address on the account's primary domain, state pending in the Fastmail API and enabled: true in x:.
2. Mail to a pending mask: delivered, state becomes enabled, lastMessageAt set (ME-7).
3. Pending mask with no mail for 24 hours: removed and tombstoned (ME-8).
4. Fastmail disabled: mail is accepted and lands in Trash (ME-5).
5. Fastmail deleted, and upstream enabled: false: mail refused at RCPT TO with the observed reply (ME-6). Both APIs read it back consistently (ME-2).
6. Destroyed mask: refused as unknown, and its address is never issued again (ME-3, ME-13).
7. emailPrefix: "shop" gives shop_…. emailPrefix: "Shop!" fails invalidProperties.
8. emailDomain the account isn't linked to fails invalidProperties (ME-12).
9. maxMaskedAddresses 2: the third fails overQuota. 0 blocks creation (ME-14).
10. The 51st create in an hour fails rateLimit (ME-15).
11. A user can't read another user's masks. An admin can. A tenant admin can only for its tenant (ME-18, ME-19).
12. (compat) INBUXA's existing masks all resolve by stored address and deliver after cutover, and read back identically through x:.

To observe

Settle against INBUXA before implementation, with a throwaway ordinary account, never by reading upstream code:

1. The SMTP reply for mail to a disabled mask, and to an expired one: code, temporary or permanent, and text.
2. What an upstream address looks like: length, characters, where a prefix goes, which domain is chosen by default. Needed only so ME-13 can't mint a look-alike, and to confirm existing addresses fit in the tombstone and lookup design.
3. What a delivered message shows: headers naming the mask, whether To is rewritten.
4. Whether mask+tag@domain delivers.
5. Whether a user can send as a mask (as an identity, or through a submission MAIL FROM), and what the recipient sees.
6. Whether x:MaskedEmail/changes exists, and whether masks can be queried by fields other than accountId.
7. Whether an ordinary user holds the sysMaskedEmail* permissions by default.
8. How many masks INBUXA already holds, over all accounts (count only), to size the compatibility test. This needs an admin, or the operator can report it.