@Michael **********
As someone under whose purview was a 3,000 user email system, I can assure you Microsoft pull this shit all the time and your IT staff have to find the offending emails and release them, and *try* and get Microsoft to whitelist the domains/addresses if possible, which is usually like pulling fucking teeth.
As I understand it, people are now starting to get emails on their Hotmail accounts, so it might be resolved.
By the way, they are not emails *from* Microsoft, they are emails *to* Microsoft, and it is (or was) Microsoft who was blocking them.