In 100% of cases, this is either because someone copied a bad regexp, or they made an explicit decision to not allow local-part wildcarding.
I have in the past normalized email addresses per the rules I know specific providers have to help people login. Did I sign up with mike+hn@ or mike+hackernews@?
I have in the past normalized email addresses per the rules I know specific providers have to help people login. Did I sign up with mike+hn@ or mike+hackernews@?