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 ...

We have people that travel and I've been annoyed a lot with some hotels using ISPs that block SMTP port 25 traffic. The solution is having to talk the users through changing the default SMTP server ...

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.