email-mirror: Add a standalone server that processes incoming email.

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.

zerver/worker/email_mirror.py

@@ -9,13 +9,7 @@ from typing import Any
 
 from typing_extensions import override
 
-from zerver.lib.email_mirror import (
-    decode_stream_email_address,
-    is_missed_message_address,
-    rate_limit_mirror_by_realm,
-)
 from zerver.lib.email_mirror import process_message as mirror_email
-from zerver.lib.exceptions import RateLimitedError
 from zerver.worker.base import QueueProcessingWorker, WorkerTimeoutError, assign_queue
 
 logger = logging.getLogger(__name__)
@@ -32,20 +26,6 @@ class MirrorWorker(QueueProcessingWorker):
         msg = email.parser.BytesParser(_class=EmailMessage, policy=email.policy.default).parsebytes(
             content
         )
-        if not is_missed_message_address(rcpt_to):
-            # Missed message addresses are one-time use, so we don't need
-            # to worry about emails to them resulting in message spam.
-            recipient_realm = decode_stream_email_address(rcpt_to)[0].realm
-            try:
-                rate_limit_mirror_by_realm(recipient_realm)
-            except RateLimitedError:
-                logger.warning(
-                    "MirrorWorker: Rejecting an email from: %s to realm: %s - rate limited.",
-                    msg["From"],
-                    recipient_realm.subdomain,
-                )
-                return
-
         try:
             mirror_email(msg, rcpt_to=rcpt_to)
         except WorkerTimeoutError:  # nocoverage