Every time I need to build a registration form I face the problem of email validation: should it be done client side, and how?
This StackOverflow tread is the best resource on the argument I found whith a lot of examples, useful links and thoughts on why you shouldn't validate email addresses client side.

Finally, even in discouraged, I used this function and I'm pretty satisfied.

function validateEmail(email){
 
   var regExp = /^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)
                 |(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]
                 {1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return regExp.test(email);
}