I use protonmail for email hosting, with a personal domain name, and the city I live in (which in turn uses Microsoft for hosting their email) always has my emails sent to spam, so I have to call every potential email recipient and have them individually whitelist me.
I’ve had a number of systems tell me my .dev email is invalid because of the TLD. Incredibly frustrating.
I have had a number of systems reject protonmail as valid. I wasnt keeping track of which ones - but there have been several, and I think most of them were logging into state/government services.
A lot of systems get their email regex wrong. Email is hard.
All digital systems by NY gov are garbage, so frustrating