On Feb 11, 2020, at 4:52 AM, Alessandro Vesely <vesely(_at_)tana(_dot_)it>
Using Courier-MTA, I'd have to devise a new add-on in order to issue these
responses using configured forward addresses. Do Sendmail or Postfix provide
for email address portability?
Postfix supports a "relocated_maps" feature, which aims to refer the
bounce recipient to a more appropriate address:
When a RCPT TO hits a listed address it looks like the response will be:
550 5.1.6 User has moved to <lookup result>
The 5.1.6 is determined in the mailer triple resolution code, which is
is SMTP independent, and 3-digit SMTP reply code is determined in the
SMTP engine, which maps replies with 550 for all addresses that
resolve to the error channel.
X.1.6 Destination mailbox has moved, No forwarding address
The mailbox address provided was at one time valid, but mail is
no longer being accepted for that address. This code is only
useful for permanent failures.
But the result is correct, Postfix is not returning a machine-readable
551 referral, it is returning a human readable 550 reject.
 By humans who can read and understand a short English phrase.
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