Update.
I was able to get the send email working by following the solution in this thread: Invalid login: 535 5.7.3 Authentication unsuccessful in MS 365 mail
However, I’m would think this workaround isn’t the right way of doing things, so I won’t mark it as a “solution” to this problem. Has anyone been able to get an MS Outlook account working through the SMPT/IMAP Oauth route? And if so, how?