During the pandemic we had a lot of problems with the confirmation email for our 5000 T.A. in the virtual campus of the university. I had to guess what was happening because I was not part of the administration team, just collecting forwarded messages form the T.A and guessing:

* Gmail: Most of the time it works.

* Yahoo: The server receives a few hundred emails per day and the other are delayed. These were confirmation emails with half an hour tolerance, if they were lucky to pass the next day they were not useful. (After a week the sending server stops retrying.)

* Hotmail: Sometimes the email is received and sometimes it just disappears. No spam folder. No bounce email. It just evaporate. (Try sending an email from hotmail to the no-reply address and cross your fingers.)

* Others: No enough data to have a good guess.