smtp_connection_helper fails with smtp: too longer line in input stream with Microsoft 365 SMTP host

[coffeegoddd]

This can be reproduced using the Microsoft 365 host with --auth login:

smtp_connection_helper \ 
--host smtp.office365.com \
--port 587 \
--auth login \
--username [email protected] \
--password ***** \
--from [email protected] \
--to [email protected]

smtp: too longer line in input stream

smtp_connection_helper is actually getting an exception response from the email server that's too long for it to read. That's the input stream too longer failure.

The cause of the problem is that [email protected] is not authorized to send mail from [email protected]:

SendAsDenied; [email protected] not allowed to send as [email protected]; STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message [BeginDiagnosticData]Cannot submit message. // continued...

[coffeegoddd]

If the --username email address/account has permission to send email from the --from email address, the smtp_connection_helper should be able to successfully send a test email.

Here is documentation on granting permission for sending emails on another user's behalf https://learn.microsoft.com/en-us/microsoft-365/admin/add-users/give-mailbox-permissions-to-another-user?view=o365-worldwide