Microsoft knows what is best for you
Outlook.com and Hotmail.com won’t receive your email message unless they want to…
https://blog.paranoidpenguin.net/2015/01/outlook-com-is-silently-discarding-email-messages/
Microsoft continues to break the protocol. One solution? Don’t email folks with outlook.com and hotmail.com email addresses. Tell them Microsoft has decided what is best.