Is it specifically the [, *, \, " symbols on the left that you're calling out, the presence of numbers on the right side, the presence of multiple periods on th

Is it specifically the [, *, \, " symbols on the left that you're calling out, the presence of numbers on the right side, the presence of multiple periods on the right side, the lack of letters on the right side, or just the lack of non-numerics on the right side? I don't think most of those rules are needed technically; I wouldn't be surprised if at least one of those rules matches someone reading this comment thread. Any one of those individually seems like a strange minor reason to pre-emptively ban a customer if it doesn't necesarrily cause technical problems. If it's only people hitting multiple of those conditions, then maybe you could use a scoring function that only rejects users that meet multiple conditions? If the reason for the validation is a character judgment instead of a technical limit, then you could add more conditions too. People with email addresses that contain accented characters probably don't even speak the same language as you, so you could save yourself some headaches by marking those invalid. If you don't like leetspeak, then I think banning the letter "x" could be a good move too on average. "+" is used by a lot of people who want to trace whoever you might be selling your email lists to, so you could ban them before they blow the whistle on anything. Email addresses containing references to political candidates probably aren't owned by savory characters, so you could mark those as invalid too... Now that I think about it, since it's not for technical reasons, then none of this is really specific to emails. You could run passwords and other user content through this validation too. If the restrictions aren't for technical reasons, then they're just arbitrary tools for blocking users. Sorry for ranting a bit. I'm just tired of how often I get mystery validation errors from services because something stupid like that it turned out they thought that english works that happen to be sql keywords like "insert" shouldn't be allowed in the subject field, or my friend with an apostrophe in his name repeatedly being turned away from software telling him his name is invalid, and the idea that these rules could be in place just to cause people like us this annoyance ticked me off.