A lot of email validation rules fail to cover valid corner cases: https://haacked.com/archive/2007/08/21/i-knew-how-to-validate-an-email-address-until-i.aspx/ h

A lot of email validation rules fail to cover valid corner cases: https://haacked.com/archive/2007/08/21/i-knew-how-to-validate-an-email-address-until-i.aspx/ https://haacked.com/archive/2007/08/21/i-knew-how-to-validat... For that matter, future developments can be problematic: Does your validation rule allow for "@gmail.com"? It seems better to warn the user that it might be invalid, rather than completely blocking some users from signing up.