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.

We are using Starlink in a remote place, which does not provide a SMTP relay server like our old (slow) cable modem ISP. A family member uses an email service she does not want to move away from that ...