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.
