Jeez, that's 100 email addresses in a million that it won't work on. Plus it's a pain in the butt. [Edit: Though I suspect that 99.99% figure was made up] Just

Jeez, that's 100 email addresses in a million that it won't work on. Plus it's a pain in the butt. [Edit: Though I suspect that 99.99% figure was made up] Just get people to enter their email twice (which filters out most mistakes where people are entering their names or somesuch), don't validate it with regex, during the signup process make sure you tell them to expect an email which they must confirm before they are added / before their account is activated. Send a confirmation email with a clickable link. If people don't get it, and the service is important to them, they'll try again or contact you through another means. (I was involved with the running of a mailing list with well over 1m double-opt-in subscribers. Less than 100 of these turned out to be invalid [Edit: yeah, that's a guess, like the OP's 99.99%], and we dealt with it easily at our end, by properly handling any bounces)