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Postfix Documentation
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Restricting the scope of "success" notifications

Just like reports of undeliverable mail, DSN reports of successful delivery can give away more information about the internal infrastructure than desirable. Unfortunately, disallowing "success" notification requests requires disallowing other DSN requests as well. The RFCs do not offer the option to negotiate feature subsets.

This is not as bad as it sounds. When you turn off DSN for remote inbound mail, remote senders with DSN support will still be informed that their mail reached your Postfix gateway successfully; they just will not get successful delivery notices from your internal systems. Remote senders lose very little: they can no longer specify how Postfix should report delayed or failed delivery.

Use the smtpd_discard_ehlo_keyword_address_maps feature if you wish to allow DSN requests from trusted clients but not from random strangers (see below for how to turn this off for all clients):

smtpd_discard_ehlo_keyword_address_maps = 

    # Allow DSN requests from local subnet only      silent-discard           silent-discard, dsn
    ::/0                silent-discard, dsn

If you want to disallow all use of DSN requests from the network, use the smtpd_discard_ehlo_keywords feature:

smtpd_discard_ehlo_keywords = silent-discard, dsn
Postfix Documentation
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