Dear Maintainer,

The instructions for integrating Mailman3 with Postfix include the
following Postfix configuration parameters:

transport_maps = hash:/var/lib/mailman3/data/postfix_lmtp
local_recipient_maps = proxy:unix:passwd.byname $alias_maps hash:/var/lib/mailman3/data/postfix_lmtp
relay_domains = ${{$compatibility_level} < {2} ? {$mydestination} : {}} hash:/var/lib/mailman3/data/postfix_domains

However, this appears to be inconsistent and can lead to a situation
where messages addressed to invalid recipients in the Mailman3 domains
are rejected with a mail "loops back to myself" message rather than a
more appropriate "User unknown". This is because local_recipient_maps is
only checked for recipient addresses in the local domain class (e.g.
domains listed in the mydestination parameter) and not for recipient
addresses in the relay domain class (relay_domains).

I believe it is more appropriate to use relay_recipient_maps instead of
local_recipient_maps together with relay_domains, e.g.:

transport_maps = hash:/var/lib/mailman3/data/postfix_lmtp
relay_recipient_maps = hash:/var/lib/mailman3/data/postfix_lmtp
relay_domains = ${{$compatibility_level} < {2} ? {$mydestination} : {}} hash:/var/lib/mailman3/data/postfix_domains

This configuration causes messages with invalid recipients in the
Mailman3 domains to be properly rejected with "User unknown".

Hello,

Debian's description follows the official documentation for Mailman3 though:

https://docs.mailman3.org/en/latest/config-core.html#configuring-mta

So you might want to report this to the upstream package.

That said, I'd appreciate that the Debian README also incudes the
default configuration values and just adds the required lines.

Balu