I've been running my mail server open relay for a while, but figured I should use whatever is considered the "right" way. This way there's less chance that mail I send is rejected, as well. This mail ...

Subject does most of the work here, but I'll expand. [edit] Maybe if I had just RTFM'd harder I would've found my answers... I think this will help, but leaving the post up in case anybody else has ...

Microsoft has shared updated requirements for SMTP relay via its Exchange Online platform. The new updated requirement is not applicable yet and is set to roll out towards the end of the year.