Mailing list features

[Sancus]

Summary

Add mailing-list-aware reply behavior for these three headers:

• Mail-Reply-To — for an ordinary private Reply, prefer this address over standard Reply-To and From.
• Mail-Followup-To — for Reply All, use this complete intended follow-up audience, while de-duplicating recipients and removing the user's owned addresses.
• List-Post (RFC 2369) — ingest the list posting address and provide an explicit Reply to List action rather than conflating list replies with ordinary Reply.

Motivation

Stormbox's current Reply and Reply All behavior is correct for ordinary mail, but it does not use mailing-list-specific routing headers. Supporting them would avoid private replies going to the list, list follow-ups omitting intended recipients, and users having to recover the posting address manually.

Thunderbird Desktop supports all three behaviors and provides a dedicated Reply to List command when List-Post is present.

Expected behavior

• Parse and persist these headers as structured address data where applicable.
• Keep ordinary Reply-To/From fallback behavior when the specialized headers are absent.
• Preserve Stormbox's existing owned-address exclusion and recipient de-duplication rules.
• Never copy Bcc into a reply audience.
• Cover ordinary Reply, Reply All, and Reply to List with focused unit and end-to-end tests.