DLP phase 3: hold for review

The hold action now holds (dlp-and-mail-flow-rules spec, §2.6), where
until now it blocked.

- At DATA a hold decision queues the message with its release a century
  off (the queue's future-release mechanism, so the stored format is
  unchanged and an older node just never sends it), transport rules
  still applied, and replies 250 Held for review. A review record under
  R/h + queue id keeps the sender, recipients, subject, size, rules and
  detector counts. The sender is told when the rule asks.
- smtp/queue/held.rs: release (each recipient due now, its next notice
  as far off as it was, its lifetime counted from the release), reject
  (removed from the queue, the sender told, with the reviewer's note),
  and expiry: the daily clean-up rejects what nobody reviewed in 7 days,
  recorded as the server's doing.
- inbuxa:HeldMessage get/set: the review queue, sysDlpReviewGet to list
  and read (preview, 64 KB of text, only when asked for and recorded as
  blobAccess), sysDlpReviewUpdate to release or reject, a reason
  required and audited by the request layer; no create or destroy;
  server-level only.
- Guards: Emails > Queue refuses to change or delete held mail; the
  sender can't unsend it.
- Privacy catalog entry for inbuxa:HeldMessage; spec §2.6 as built.

Tests: mail_rules_tests gains the whole flow (held and listed with
counts, sender notified and nothing delivered, queue and unsend
refused, preview recorded, reject needs a reason and tells the sender
the note, release delivers, expiry returns it, decisions audited with
reasons). smtp inbound, system_tests (after one BlobNotFound in
antispam, the known flake, then clean), features and common unit tests.

crates/smtp/src/queue/held.rs

@@ -0,0 +1,177 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! inbuxa: mail held for review (dlp-and-mail-flow-rules spec, §2.6):
+//! releasing it, rejecting it, rejecting what nobody reviewed in time, and
+//! the notices the sender gets.
+//!
+//! A held message sits in the queue with its release [`HOLD_SECONDS`] off.
+//! Releasing it undoes exactly that: each recipient due now, its next
+//! notice as far from now as it was from its retry, its lifetime counted
+//! from the release.
+
+use crate::{
+    queue::{Message, MessageWrapper, Status, spool::SmtpSpool},
+    reporting::send::MtaReportSend,
+};
+use common::{
+    Server,
+    config::smtp::queue::{QueueExpiry, QueueName},
+    ipc::QueueEvent,
+};
+use inbuxa_features::{
+    audit::{Action, Actor, Outcome, Record, Target},
+    mailflow::held::{self, HOLD_SECONDS, Held, KEEP_DAYS},
+};
+use mail_builder::{
+    MessageBuilder,
+    headers::{HeaderType, address::Address},
+};
+use store::{ahash::AHashSet, write::now};
+
+/// Puts a held message back on its way. False when it's no longer queued.
+pub async fn release(server: &Server, queue_id: u64) -> trc::Result<bool> {
+    let Some(archive) = server.read_message_archive(queue_id).await? else {
+        held::delete(server.store(), queue_id).await?;
+        return Ok(false);
+    };
+    let mut message: Message = archive.to_unarchived::<Message>()?.deserialize()?;
+    let prev_events = message.next_events();
+    let at = now();
+    let mut modified = AHashSet::new();
+    for (idx, rcpt) in message.recipients.iter_mut().enumerate() {
+        if !matches!(rcpt.status, Status::Scheduled | Status::TemporaryFailure(_)) {
+            continue;
+        }
+        let notify_gap = rcpt.notify.due.saturating_sub(rcpt.retry.due);
+        rcpt.retry.due = at;
+        rcpt.notify.due = at + notify_gap;
+        if let QueueExpiry::Ttl(ttl) = rcpt.expires {
+            rcpt.expires = QueueExpiry::Ttl(
+                ttl.saturating_sub(HOLD_SECONDS) + at.saturating_sub(message.created),
+            );
+        }
+        modified.insert(idx);
+    }
+    let saved = MessageWrapper::new(message, queue_id, QueueName::default())
+        .save_registry_changes(server, prev_events, modified)
+        .await;
+    held::delete(server.store(), queue_id).await?;
+    let _ = server.inner.ipc.queue_tx.send(QueueEvent::Refresh).await;
+    Ok(saved)
+}
+
+/// Takes a held message out of the queue and tells its sender, with the
+/// reviewer's note if there is one. False when it's no longer queued.
+pub async fn reject(server: &Server, record: &Held, note: Option<&str>) -> trc::Result<bool> {
+    let removed = match server
+        .read_message(record.queue_id, QueueName::default())
+        .await
+    {
+        Some(message) => message.remove(server, None).await,
+        None => false,
+    };
+    held::delete(server.store(), record.queue_id).await?;
+    let mut text = format!(
+        "Your message \"{}\" to {} was held for review under this server's rules, and wasn't sent.\r\n",
+        record.subject,
+        record.recipients.join(", ")
+    );
+    match note {
+        Some(note) => text.push_str(&format!("\r\nThe reviewer's note: {note}\r\n")),
+        None => text.push_str(&format!(
+            "\r\nNobody reviewed it within {KEEP_DAYS} days, so it was returned.\r\n"
+        )),
+    }
+    notify(
+        server,
+        record,
+        &format!("Not sent: {}", record.subject),
+        text,
+    )
+    .await;
+    let _ = server.inner.ipc.queue_tx.send(QueueEvent::Refresh).await;
+    Ok(removed)
+}
+
+/// Tells the sender their message is held (when the rule asks).
+pub async fn notify_held(server: &Server, record: &Held) {
+    let notices = record
+        .rules
+        .iter()
+        .map(|r| r.notice.as_str())
+        .collect::<Vec<_>>()
+        .join(" ");
+    let text = format!(
+        "Your message \"{}\" to {} is held for review under this server's rules: {notices}\r\n\r\n\
+         It will be sent if a reviewer releases it, and returned otherwise within {KEEP_DAYS} days.\r\n",
+        record.subject,
+        record.recipients.join(", "),
+    );
+    notify(
+        server,
+        record,
+        &format!("Held for review: {}", record.subject),
+        text,
+    )
+    .await;
+}
+
+async fn notify(server: &Server, record: &Held, subject: &str, text: String) {
+    let domain = record
+        .sender
+        .rsplit_once('@')
+        .map_or("localhost", |(_, d)| d);
+    let from = format!("postmaster@{domain}");
+    let message = MessageBuilder::new()
+        .from(Address::new_address(Some("Mail review"), from.clone()))
+        .to(Address::new_address(None::<String>, record.sender.clone()))
+        .subject(subject)
+        .header("Auto-Submitted", HeaderType::Text("auto-replied".into()))
+        .text_body(text)
+        .write_to_vec()
+        .unwrap_or_default();
+    server
+        .send_autogenerated(from, [record.sender.as_str()].into_iter(), message, None, 0)
+        .await;
+}
+
+/// Rejects every held message nobody reviewed in time (§2.6), each
+/// recorded as the server's doing. Returns how many.
+pub async fn expire(server: &Server) -> trc::Result<usize> {
+    let at = now();
+    let mut count = 0;
+    for record in held::all(server.store()).await? {
+        if !record.is_expired(at) {
+            continue;
+        }
+        reject(server, &record, None).await?;
+        count += 1;
+        server
+            .audit_note(Record {
+                at: at * 1000,
+                actor: Actor::system("DLP"),
+                via: None,
+                remote_ip: None,
+                action: Action::Destroy,
+                target: Target {
+                    kind: "inbuxa:HeldMessage".into(),
+                    id: Some(record.queue_id.to_string()),
+                    name: Some(record.subject.clone()),
+                    account_id: record.account_id,
+                    tenant_id: record.tenant_id,
+                },
+                changes: vec![],
+                details: Some(format!(
+                    "Rejected: nobody reviewed it within {KEEP_DAYS} days; the sender was told"
+                )),
+                reason: None,
+                outcome: Outcome::success(),
+            })
+            .await;
+    }
+    Ok(count)
+}