Well... having been asked to move the company from Google to 365, I am now facing the same issue as many others.
OST 1.15.4 (I do need to update, but this should work, right?)
App password correctly set up, IMAP enabled, POP enabled etc.. but nothing allows authentication to work.
If I disable authentication for SMTP, then of course it sends, but as expected the result is in the receiver's spam/junk so that's no good.
I've gone through the options recommended here, and still not working as it should. Just about at my wits end - any further ideas or suggestions before I jump off the ledge?