Suppressing (some) bounce messages

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Suppressing (some) bounce messages

Fredrik Sandström
I run a gateway that delivers mail to a few different places, including
two Exchange servers. Now, some users on one of the Exchange systems
have managed to block certain senders, in a way that makes Exchange
reject such messages with "554 5.1.0 Sender denied". The Exchange admins
are trying to figure this out; in MOST cases blocking a sender results
in messages being placed in a Junk folder, which is the behaviour we
want. But sometimes, this happens. Anyway, getting Exchange to behave
consistently is thankfully not my concern, nor the topic of this list.

So, if the Exchange admins cannot figure this out, we will become a
backscatter source unless I can prevent Postfix from generating
non-delivery notifications in these cases. To be precise, when a
particular host (the Exchange server) is rejecting a message, I want
Postfix to simply ignore that instead of sending a bounce. How can I do
that? Or should I do something else instead? (Naturally, I don't want to
suppress ALL non-delivery notifications, only those that arise in this
particular way.)

Ideally, I would want Postfix to reject those messages that Exchange is
going to reject, but I don't see how that would be possible. (Exchange
rejects after DATA.)