well, there was this, a while back: "experiencing a critical and recurring email delivery issue affecting recipients at outlook.com, live.com, hotmail.com, and msn.com"

"Emails to Outlook.com rejected due to a fault or overzealous blocking rules"

https://www.theregister.com/2026/03/04/users_fume_at_outlook...

discussed here: https://news.ycombinator.com/item?id=47246044