Agreed. Anyone that decides to come up with a "common sense" subset of emails to allow is more likely to accidentally block some perfectly normal addresses than

Agreed. Anyone that decides to come up with a "common sense" subset of emails to allow is more likely to accidentally block some perfectly normal addresses than actually accomplish something useful. I remember I once made this point in an HN thread, and I got a very dismissive and overconfident reply by someone who posted a regex that turned out to block my own email address (which had a number immediately before the @ symbol). Writing code that doesn't need to exist that has a possible failure mode of not letting someone sign up at all is just a bad decision. If you're going to write that code, either go through the effort of getting it completely right or soften the failure mode. If you really think the user is somehow mistyping their email with special characters or an unusual TLD, then you could show them a non-blocking warning message.