I’m just going to jump in here as i did this with a RUTX11 with an o365 setup and got stuck for a week. It came down to the fact that the email account we were being asked to used required MFA. The error is not very apparent. This is a setting in AZURE that you can turn off for that specific user. (You can also do some other crazy things if your concerned about security. e.g. make sure your using an obsurdly long password and you can in Azure lock the use of the account login to a MAC or IP of the Tetonika device). Granted for all of this you need admin access on the Azure side.
I mention this only as I got stuck on this, and only after i installed thunderbird on another laptop and setup the email address and SMTP server did I notice that this was the problem.