[caf]

You are assuming that only one RCPT is being sent in the session.

(What does happen with your reverse-proxy when someone opens up a session and sends two mails, to two different customers?)

[capt_hotpants]

Currently, this happens:

   if hostname != target {
      downstream.Write([]byte("452 Different domain, please reconnect and deliver separately.\r\n"))
      continue
   } else {

As a side-effect, email to the secondary domain is slightly delayed.

It is a full proxy, in the sense that it sees all of the traffic, so technically it could de-multiplex and spool to two different targets at the same time for the duration of the current email.. Hasn't been a noticeable problem so far. But it would be nice to add at a later point in time. If/when someone complains, probably.