I found it maddening that I couldn't whitelist a sender (MS Outlook web, Gmail); well I could, but they still blocked the messages.

In my case, it was reportedly (for MS) an IP associated with mine (same hosting provider) had previously been used to send spam.

My domain is decades old, never sent any spam, and I whitelisted it .. but nope, my host wasn't perfect.

This was some time ago now, but it looks like they've still not adopted proper whitelisting.