Hi John CFS,
To narrow down this issue, we have some things to confirm with you:
1. Whether the sender’s server is an Exchange server?
2. Whether other users in the sender’s server can send mails to the Office 365 account?
If others user on the sender‘s cannot send mails to this Office 365 account, please check the settings of send connector on the sender’s server.
Regards,
Kyle Xu
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