"Now you have two problems." Seriously, I can't think of a single good reason why you would want to check whether an email address is "valid". What you should

"Now you have two problems." Seriously, I can't think of a single good reason why you would want to check whether an email address is "valid". What you should be concerned about is whether or not the address works (and usually, can/does the person who just signed up actually read and reply to email to that address). Hypothetically, if an invalid address works (due to bugs in mail systems) -- then it works, and the only problem with accepting such an address is that the bugs might get fixed. If an address is valid, there's no reason to assume that it will work or that it belongs to the person who signed up. It isn't even a good way of detecting typos; transpose two characters in an email address and it will most likely still pass your validation.