I cannot tell you how many times I have encountered a person that asks for my email and then looked at me confused when it didn't end in gmail, outlook, or something, and then ask me for another email, as if only that would be good enough.
ON the other hand I had a website with a ccTLD and one guy kept trying to add .com to it - his own country's ccTLD!
Usually, when that sort of thing happens, I just choose to take my business elsewhere or use my time in a different way. However it really ups the ante when a nation-state refuses entry based on that category of technical incompetence.
Also, a lifetime ago, had Hurricane Electric (https://he.net) refuse to accept a sign-up because... I only had one MX record. I added an additional record pointing the same IP, but they weren't happy.. So I allocated an additional ipv4 address to the same server, just to satisfy them.
It's a losing battle.
HE provides professional network services and probably, perhaps even contractually, requires you to have two different ways to reach you. Everyone at that level has quick ways to get in contact with all their business partners, usually a phone number that's active 24/7.
edit: Changed domain based on ButlerianJihad's suggestion
https://en.wikipedia.org/wiki/.example
A real domain, for all your fake purposes!
"Do you mean 'myname@foo.example.com'?"
And most of the confusion in person seems to be down to my choice of TLD: .es and .io (I'm neither from Spain nor the Chagos Archipelago).