The IMAP poll loop previously downloaded the entire message body for
every UNSEEN message before running any filter (self-sent, SPF/DKIM,
allow-list), and only learned the UID by parsing it back out of that
fetch response. Rejected messages stay unseen and get re-fetched in
full on every subsequent poll.
Switch to UID SEARCH (UIDs come back directly, no per-message fetch
needed to learn them) so already-processed UIDs are skipped before any
network fetch, then fetch headers only to evaluate every filter — the
full body is downloaded only for messages that pass every check and
are actually delivered. No behavior change: filter outcomes and
\Seen semantics are unchanged.
