[TODO]: Sieve - Access original recipient address before alias resolution

[polarathene]

Description

I touched on this while adding these notes and reference them with more detailed insights shared in this discussion (focused on resolving the recipient for a catch-all alias).

This issue is more for visibility/discovery, but if a proper solution is identified we could probably document that in our sieve docs page, and consider official feature support on the master.cf side at least if the modifications are non-trivial.

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For now it is believed that the lmtp service + transport in /etc/postfix/master.cf just needs to add some extra flags, and for /etc/postfix/main.cf to rate limit delivering mail per recipient to Dovecot instead of delegating Dovecot to manage. Doing so would make additional headers Delivered-To + X-Original-To available to Sieve:

user-patches.sh:

#!/bin/bash

# Required for the Postfix to send with original recipient headers for Sieve to use:
# Change from default limit of 50 to 1,
# Postfix will send mail with multiple recipients to Dovecot one at a time
# instead of delegating to Dovecot to manage all at once:
postconf lmtp_destination_recipient_limit=1

# Modify the `lmtp` service to use the `lmtp` transport command with extra `flags=DO` arg:
# This will enable both `Delivered-To:` and `X-Original-To` headers for Sieve to have access:
postconf -F 'lmtp/unix/command=lmtp flags=DO'

Current queries:

• There's potentially a concern with other services like Amavis integration with Postfix possibly interfering with the expected recipient value being resolved before the feature config above would be handled? Requested user tries without anti-spam service enabled to confirm.
• User in discussion only mentioned trying with X-Original-To, whereas Delivered-To may be what they're after as it's the envelope recipient address which shouldn't be altered AFAIK (even with Amavis involved).

[polarathene]

The linked discussion has had some recent feedback of the advice I provided working well all of a sudden. While another user said they didn't need any of it, but has not responded yet with context requested.

If someone gets around to investigating this, it'd be good to contribute several test cases for those different contexts.