[ubernostrum]

Maybe you don't need to send email right away, but want to store the address in case you need to get in touch with the user?

Sending one email per signup can be problematic depending on the volume of signups. Sending email only when absolutely necessary can help with that.

[ankhmoop]

If you really care, use a real standards-comformant address parser, most languages have at least one -- Java does. Otherwise you're just wasting your time, and the time of any users you hose with your amateur-hour validation.