troubleshooting Question

email bounces with "Delivery failed" email "Unknown user".

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jbmsystems asked on
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My emails are bouncing when I send them to known addresses with results like
"Delivery failed" email  "Unknown user".
The user exists and if I use an alternate account it works  (
My customer says that I am not being blocked, but this is happening with MANY people that I communicate  with regularly.  

I cannot reply to mails and must often copy the mail and replay via the alternate.


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