I know this was an issue many years ago -- now I've run across a customer that we want to set this up for (they got bought out, and the parent company migrated all their email to O365, and the folks I deal with have no access to the admin controls in O365) -- and no matter what I do, I cannot send firewall notifications out via a configured O365 relay (they require authentication). I do have other customers that I do manage, and in those cases we helped them setup relays (by static IP) that did not require authentication, and that works.
I suspect that (somehow, it's been a very long time) that Sophos has not fixed this issue yet (if you use authentication, you have to have TLS/StartTLS enabled with O365) with Sophos Firewall. IIRC, it had to do with them using MD5 which Microsoft rejects. I will be starting a support case on this, but checking here first to see if any of you have had a different experience recently.
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