I installed Thunderbird and typed in my Office 365 email information to connect my account. Thunderbird found the correct IMAP settings, which I verified on Microsoft's website. It proceeds to open a browser so I can authenticate my account. After that succeeds it starts downloading all of my messages & folders. I thought everything went fine until I attempted to send a message. I get the following error: Login to server smtp.office365.com with username me@mydomain.com has failed. It gives me the option to enter a new password, which doesn't make sense since it's oAuth2 but I created an app password to see if that would work, and it didn't. The only other options are cancel & retry which don't help either. I thought maybe something went wrong with the authentication process so I uninstalled thunderbird and started again with the same results. I've tried many other server settings, but nothing works, so I'm here. Current Settings: Server Name: smtp.office365.com User Name: me@mydomain.com Authentication Method: oAuth2 Security: STARTTLS Port: 587 I've tried: Using Port 465 / SSL/TLS Using Port 465 / STARTLS Using Port 587 / SSL/TLS Changing Auth method to Normal Password and then using app-specific password. Uninstall / Resintsall One final thing to note: I followed the same process with my fastmail.com account, which worked flawlessly. I can send & receive fine.