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Sophos Central Email rejecting Distribution Lists

My ISP sends voicemail as an attached WAV file for any extension.  Each extension is tied to an email address.

When the associated email address is a Microsoft Distribution List, Sophos reports that the user is unknown and the message never gets delivered.  Somehow the Sophos relay ( knows that the recipient is a DL and simply fails:

smtp sendmail[14278]: 342KKrXE014275: to= <>, delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=126720, [], dsn=5.7.4, stat= User unknown

The same ISP can mail directly to the DL.

My outside email, such as Gmail, can send directly to the DL.

Any email address, internal or external, can be successfully associated with an extension.

None of this makes sense.  It would seem that there's some kind of handshake going on between Sophos and Microsoft 365 (my email service) and perhaps headers are being changed?

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