A few years ago, in my university we have a big problem at the beginning of the semester to contact ~10K students, in particular when they register to our Moodle platform and the server sends them a message.

Gmail was usually ok.

Yahoo had some max messages per day.

But Hotmail/Live/Outlook/whatever just made the messages disappear, no spam folder, no bounce, just disappear. We had some success telling the students to send us a message from their Hotmail/Live/Outlook/whatever address half an hour before registration. This adds our address to some special secret list for that account, and our later messages (usually) reach them. (It may fail. It may fail. IWOMM. YMMV.)