Server-side mail rules that actually run: Dovecot Sieve for sorting, forwarding, and vacation replies

A client of ours had a Thunderbird rule that filed invoices into an Accounting folder. It worked great — on the desktop. On the phone, the same invoices piled up unread in the inbox, because the rule only fires when Thunderbird is open and syncing. Then the laptop got rebuilt, the rule config didn't survive the migration, and now there were three devices with three slightly different sets of filters, none of them authoritative. This is the normal failure mode of client-side mail rules: they run in the wrong place, at the wrong time, and they drift.

Server-side Sieve fixes this by construction. The rule executes exactly once, at delivery, inside Dovecot's local delivery path — before the message ever lands in a mailbox. A phone opening IMAP, a webmail session, and a desktop client all inherit the same already-sorted inbox, because the sorting happened on the server hours before any of them connected. Put your mail logic in Sieve, keep it in one place, and test it before it goes live. The rest is detail.

Where Sieve runs in the delivery path

Sieve is not a daemon you poll. It's a plugin that runs synchronously in the local delivery agent. The chain looks like this: Postfix accepts the message over SMTP, then hands it to Dovecot over LMTP (or the older LDA binary). Dovecot's Pigeonhole plugin evaluates the Sieve script, performs whatever actions it dictates — file, flag, redirect, discard — and only then commits the message to storage. If nothing matches, the implicit keep drops it into INBOX.

There are three script layers, and understanding their order is 90% of using Sieve well: