I used to have an email address with a three-letter username @eh.net, which was actually surprisingly annoying to use. If you ever gave the address to someone they'd be very confused at how short it is.
They needed information from a third party. I gave the accountant the third party's email address, firstname@lastname.co.
The transaction was taking a while to get resolved. After repeated followups, the accountant finally told me that he was having a hard time getting a hold of the third party. Was I sure that their address was firstname@lastname.com?
Like... what the heck. We're all corresponding over email. You could have literally clicked the email address in the email that I sent you. But instead, somehow you've mangled it and are confused that it doesn't work?
That was instructive for me. .com TLDs only going forward!
"No, no. Not everyone has to use Gmail!"