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Microsoft 365
Important
General
- Setup a Catch All email
Troubleshooting
invalid sender
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.
- login office 365 admin portal
- go to Microsoft 365 security at https://security.microsoft.com/restrictedusers
- if there is any user blocked, select it and click [Unblock]
- it may take an hour to be effective