Axiom 1: If you're going to gather information on your users, make sure it's valid information.
If you want to validate your user's email you need to check for a few things. The @ symbol, at least one period, at least one character before the @ symbol, at least one after before the period, and at least one after the period. Here are two server-side scripts which will help you validate email!
Ian Stallings JScript/Regular Expression Email Validation Routine
Ben Durbin's Email Validation Routine
Joao Vieira's Email Validation Routine