[nemetroid]

> Both Microsoft and Google seem to do it just fine

Microsoft sends me DMARC reports saying "yes, everything was accepted 100%, all good". The delivery logs on our end look good as well. However, they silently drop a large portion of messages with a Hotmail destination.

[ZeroConcerns]

Yeah, there is a whole layer of Rube Goldberg-esque nonsense between the public Microsoft SMTP servers and the actual destinations that not even the highest levels of their support seem to properly understand.

Like: if you truly want to ensure delivery to an Office 365 tenant, EWS is pretty much the only option. Anything else will have random gaps, even after the tenant themselves have begged everyone they could find to let that particular sender, domain, IP, and everything through no matter what...