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Outlook 2016 IMAP error: 550 5.1.0

I have a client with an issue with Outlook 2016 on Windows 10.  Both should have current updates.  When Outlook does a send/receive he gets an error message: Sending reported error (0x800CCC0F): 'The connection to the server was interrupted.  If this problem continues, contact your server administrator or Internet service provider (ISP).  The server responded: 550 5.1.0 <> sender rejected: invalid sender domain'.

This message follows the brief one: "Sending message 1 of 1".  If I create a new email, it will change to "Sending message 1 of 2" and then get stuck at "Sending message 2 of 2".

As far as I can tell, the email gets delivered.

I presumed that there would be an email "stuck" in the Outbox, but it is empty.  This happened before where there were about a dozen emails "stuck" in the same fashion, but the Outbox was similarly empty.

I have deleted the OST file and had it recreated a couple of times.  It worked for a bit and then the symptom returned.  I deleted the profile and recreated it with the same results.

The email is through his domain hosted at Comcast.

I set this up on a different computer (Windows 7, Outlook 2013) and am not seeing the symptom on it, but I didn't test for very long.

My best approach at this point is to recreate the OST file again and have him send emails one at a time until he sees what one is causing the problem.  I'd appreciate a better suggestion.

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8/22/2022 - Mon