Using postfix to handle the incoming email gateway complicates things
a great deal:
- It cannot verify that incoming email addresses exist in Zulip before
accepting them; it thus accepts mail at the `RCPT TO` stage which it
cannot handle, and thus must reject after the `DATA`.
- It is built to handle both incoming and outgoing email, which
results in subtle errors (1c17583ad5, 79931051bd, a53092687e,
#18600).
- Rate-limiting happens much too late to avoid denial of
service (#12501).
- Mis-configurations of the HTTP endpoint can break incoming
mail (#18105).
Provide a replacement SMTP server which accepts incoming email on port
25, verifies that Zulip can accept the address, and that no
rate-limits are being broken, and then adds it directly to the
relevant queue.
Removes an incorrect comment which implied that missed-message
addresses were only usable once. We leave rate-limiting to only
channel email addresses, since missed-message addresses are unlikely
to be placed into automated systems, as channel email addresses are.
Also simplifies #7814 somewhat.
This timeout needs to be short enough that we don't drop the RabbitMQ
connection. Also drop the offending message (by returning with no
further exception) so we don't hit a head-of-queue failure situation.
Ideally, the parser would just be lightning-fast, so this would never
happen.
