In general terms I absolutely agree with you. However, the subtleties might have more to do with ensuring that the intent of the transaction is adhered to rath

In general terms I absolutely agree with you. However, the subtleties might have more to do with ensuring that the intent of the transaction is adhered to rather than not. The first assumption is that your intent is to capture accurate user information and the user's intent is to give it to you. With that established, it is a good idea to apply sensible measures at your end in order to ensure that the data is accurate. Then there's the reduction of junk signups and the like. This is a case where your user doesn't necessarily have to know that you are validating. You can simply tag the signup as potentially invalid in the database and require human inspection or discard it if there's enough information to do so. In other words, the validation happens in the server and with no feedback to the client. The RFC822 regex expression will pass something like this as valid: joe @example.com joe@ example .com joe@example.ccom joe@example.com- joe@example.com---------------- joe@example.com///// You can check it yourself here: http://mythic-beasts.com/~pdw/cgi-bin/emailvalidate http://mythic-beasts.com/~pdw/cgi-bin/emailvalidate I haven't done exhaustive testing on that particular expression. I don't really know where it fails. And that's part of the problem. If the "contract" is that both parties want this information to be correct the user couldn't possibly be annoyed if you point out a legitimate error in the email address. You can prevent a situation where someone fills out a form, clicks "send" and goes away thinking that the signed-up when they actually didn't because they made a mistake entering their email address. What are you going to do? Send them an email? Obviously, if they enter jjoe@somedomain.com instead of joe@somedomain.com The only hope you have to catch it is to make them enter it twice and hope they don't make the same mistake twice. This is ugly and bad for such things as landing page signup forms. People don't generally respond well to having to type their address twice. So, yes, even with validation you are going to loose a few. What you are going to catch are cases where the email is entered with detectable mistakes: jot dot@somedomain.com jotdot@@somedomain.com jotdot@somedomain.com. jotdot@ somedomain.com jotdot@somedomain..com jotdot@ somedomain.comm etc. instead of jotdot@somedomain.com With a good email validation approach --which includes DNS checks-- you can catch most of these and alert the user. I don't think this is a bad idea at all. While it is true that that super-large regex expression seems to validate most addresses correctly, there's a voodoo out there in the realm of regex for email validation. Buyer beware.