I have been self-hosting for about 25 years. I remember the protection.outlook.com issue. Once there was an issue with a bank that tried to do encryption, but used an expired certificate. But once I told them what the problem was, and that it was a problem for paying customers, they actually fixed it.

Being able to check the server log can be very useful. E.g. to tell someone that their mail was delivered to a served using their domain name, with that IP-address at that time.