Outgoing email fails with an error when using Office 365 mail server. Error found in the logs 

"Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 16.55847:85150000, "

This mostly happen when the clutter feature is enabled for the mail user. 

Refer to document https://docs.microsoft.com/en-us/exchange/troubleshoot/email-messages/mapiexceptionnotfound-ndr.