Happened to me. My Yahoo account with an underscore didn't work but another account with only letters did. Never thought that could be the problem.
Well, the error message could be improved. If it said "this email address contains invalid characters" instead, it should be much easier for users to figure out the problem.
Making this change should not take more than a minute.
Ideally, the email address check would be reworked entirely. After all, unicode email addresses do exist these days. But changing the error message would be a good quick fix at least.
Thankfully this was forwarded to me!
I must be as incompetent as some have said in this thread. Took me more than a minute to update it
Yes, the email verification code was at least 18 years old. I updated the verification, and put in a bit more specific message.