The entire "calling up and having to explain the username" thing is few and far between. Had that conversation in person and over the phone dozens of times, at most I get a little ask to verify I spoke correctly. Customer support doesn't care. They have people calling them up with an email address of "420hotcock69 at something dot tld". The entire "your company name at silly domain dot tld" generally doesn't phase them.
Mispelled accounts? Rarely happens. Copy and paste the domain. Use a password manager. The only time I have trouble logging in is when I can't remember if I used social auth or created an account.
As for getting email accounts purged? Don't bother. Stop using it for whatever legit reason. Then set a filter to mark all email to that user@ to be sent to spam/deleted. Problem solved.
The ONL time I've had issues was a few random systems that had funky rules for verifying fake email addresses. Oddly they sometimes look for their own domain name in the email address. So I can't use the exact domain name at those.