In my experience Office 365 is even harder to deliver to.

Yep, this is my experience as well.

I’ve had some issues with google, but at least they tend to tell me the messages are being rejected.

Microsoft will give me a 250 message, and then route the message to /dev/null.

That’s contrary to the RFCs, and really annoying. Since it doesn’t end up in Junk, the receiver can’t say ‘not junk’, and since it doesn’t bounce, the sender thinks it has been sent.

I’m signed up for Microsofts junk mail reporting, and when this happens the UI shows no issues with my ip, and doesn’t admit to any e-mail filtering. The only way I can detect it is by sending messages to my test accounts, or waiting for users to yell.

Fwiw, anyone else who runs in to this scenario, expect your first support ticket with microsoft to be rejected. Keep responding to it. On the second or third try they might end up removing the silent ban.