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Email sending fails with sender domain unknown

Superuser
Superuser
Posts: 9,888
Thanks: 1,252
Fixes: 71
Registered: ‎06-04-2007

Email sending fails with sender domain unknown

During the last few days a friend has experienced two failures to send when emailing me both reported by relay.plus.net as "Domain of sender address <...> does not exist ".
Quote
A message that you sent to the following recipient could not be delivered due to a permanent error. ** The remote server 212.159.8.200 responded with: ** me@myf9domain 550:#5.1.8 Domain of sender address <friend@theirpndomain> does not exist This message was created automatically by mail delivery software on the server avasout04.

"theirpndomain" does not have an A record but does have an MX record which should suffice. Indeed it worked before, between and after those two instances. My friend uses Outlook 2010 on Windows 7.
I'm told this is not a known problem. Has anyone else experienced it?
David