Issue Dovecot 2.4.1-4 and 2.4.2 silently ignore administrator-scoped mailbox { sieve_script } blocks for IMAP events

[stephan.kurz]

Server operating system version

Ubuntu 24.04 x86_64

Plesk version and microupdate number

18.0.77 #2

PRODUCT, VERSION, OPERATING SYSTEM, ARCHITECTURE
Product version: Plesk Obsidian 18.0.77.2 (also reproduced on 18.0.74.3)
OS version: Ubuntu 24.04 x86_64
Dovecot: plesk-dovecot-core 2.4.2 (also 2.4.1-4 on 18.0.74.3)
Pigeonhole: plesk-dovecot-pigeonhole 2.4.2

PROBLEM DESCRIPTION
An administrator-scoped sieve_script placed inside a mailbox { } block, using the documented Dovecot 2.4 syntax, parses cleanly and loads without errors. Dovecot starts, systemctl reports active, doveconf -n shows the block. IMAP COPY/APPEND events on the target mailbox occur normally. But the sieve_script inside the mailbox block never fires — no execution log entries, no script side effects.

Goal: use imap_sieve for rspamd Bayes feedback — move INBOX → Spam fires learn_spam, move Spam → INBOX fires learn_ham. Standard pattern documented at imap-sieve | Dovecot CE

STEPS TO REPRODUCE
1. Pre-compile sieve scripts at /etc/dovecot/sieve/report-spam.svbin using sievec. Verify the .svbin is readable by the imap process.
2. Add to /etc/dovecot/conf.d/99-custom.conf:

protocol imap {
mail_plugins {
imap_sieve = yes
}
}

sieve_plugins {
sieve_imapsieve = yes
sieve_extprograms = yes
}

sieve_extensions {
vnd.dovecot.pipe = yes
}

namespace inbox {
mailbox "INBOX.Spam" {
sieve_script learn_spam {
type = before
cause = copy
path = /etc/dovecot/sieve/report-spam.svbin
}
}
}

3. doveconf -n — no errors, block appears in output.
4. systemctl restart dovecot — starts cleanly, active (running).
5. From a real IMAP client (tested Thunderbird, Apple Mail), move a message from INBOX to INBOX.Spam.
6. Observe /var/log/maillog — IMAP COPY event visible, but no sieve execution entries. No script side effects.

Also tried mailbox Spam { ... } (namespace-internal name, no INBOX. prefix) — same silent no-op. Note: doveadm copy does NOT reproduce — imap_sieve fires only on real IMAP client sessions.

EXPECTED RESULT
The sieve_script inside the mailbox block executes on IMAP COPY events to the matched mailbox, per the Dovecot 2.4 IMAPSieve documentation. The script should run, produce log entries, and invoke the configured pipe command.

ANY ADDITIONAL INFORMATION
Same symptom reproduces independently of Plesk packaging:

• Plain Debian Trixie + Dovecot 2.4.1-4 (same build as Plesk 18.0.74.3): 2.3 -> 2.4 sieve scripts - dovecot - dovecot.org — user attempting the same pattern hit an adjacent ABI symbol error. Zero replies in 357 days.
• ISPConfig on Debian 13 + Dovecot 2.4.1: Issue with Rspamd learning (IMAPSieve) on Dovecot 2.4.1 (Debian 13) — identical silent no-op symptom. ISPConfig lead developer had no solution.

This suggests a Pigeonhole 2.4.x settings-handling issue for administrator-scoped mailbox blocks, not a Plesk packaging issue. But Plesk is the update path for this server, and any fix will need to land via plesk-dovecot-pigeonhole package updates.

Distinct from PPPM-15312 (Dovecot crash when imap_sieve=yes, triggered by mail_attachment_detection_options default change in 2.4.2, fixed in 18.0.76 Update 2). That is a separate bug — it's a crash; this is a clean-startup, silent-no-op-at-event-time bug.

The 2.3-era imapsieve_mailboxN_* keys are hard-rejected on Dovecot 2.4 as expected ("Unknown setting"), confirming the 2.3 syntax is not a fallback.
Config has been reverted on this server. Happy to re-enable temporarily for any reproduction steps Plesk staff want to verify. doveconf -n output available on request.

EXPECTATIONS FROM PLESK SERVICE TEAM
1. Confirm reproduction on a Plesk Obsidian 18.0.77.2 + Ubuntu 24.04 server.
2. Track as a separate bug from PPPM-15312.
3. Coordinate with Dovecot / Pigeonhole upstream if root cause is confirmed in plugin settings handling.
4. Consider bumping plesk-dovecot-pigeonhole to 2.4.3 in a future Plesk release — upstream 2.4.3 has settings-handling fixes that may address this; worth retesting.