My first experience with MS Exchange long time ago was that the team responsible for the infrastructure (company with more than 100k employees) committed to reboot the server once a week, because otherwise it would blow up.

So yes, this is a long story.

Oh, my first contact with Exchange was discovering that the recently updated server couldn't read any of the backups on the proprietary format of the pre-update version of it. It seemed to be a common enough occurrence, because the email people just shrugged and started hacking the backup. I don't think that group was ever capable of restoring any Exchange backup, normally because of Exchange's problems.

But that was a long time ago. From what I hear, things are different now.

Sounds like unchecked IIS SMTP transport logs but hey it's been years since I maintained an on-premises Exchange server

And icloud too. They are very sensitive to ipv6.

In case of icloud, I attribute it to the Proofpoint spam filtering system, which also sell service to ups.com.

And even gmail, but at least gmail accept the email, then just flagged it as spam.