Сегодня ночью произошло это эпохальное событие, - в postfix появилась поддержка DSN (RFC 3461-4). Вот письмо Винемы:

FYI, I have now all full DSN support implemented in Postfix (RFC
3461-4). This means that Postfix will be a "complete" MTA by the
time of the Postfix 2.3 release.

DSN support gives an SMTP client the ability to request:

- When notifications are sent: never, success, delay, failure. In
  the case of success, notification is sent when mail is delivered
  to its final destination, when it is relayed to an MTA that does
  not have DSN support, or when it is expanded by an alias.

- What is sent: the headers or the full message.

In addition, the client can specify:

- An envelope ID that is reported in the notifications. This can
  also be useful as a unique message identifier.

- An original recipient address that can be a non-RFC822 address,
in case mail was gatewayed from a non-Internet environment into
the Internet environment.

As noted by Claus Assman, DSN notification can reveal a lot of
inside information. This is especially true for "success" notifications;
to get an idea, use Postfix's "sendmail -v" feature. It usually
gives away more information than bounced mail, which can already
be a lot.

It is therefore possible that Postfix will restrict the use of DSN
support by default to only trusted networks.

        Wietse