The only really accurate way of distinguishing real, valid email addresses from invalid ones is to send mail to it. What counts as an email is surprisingly convoluted ("John Doe" " actually is a valid email address), and you most likely want the email address to actually send mail to it later. After it passes some basic sanity checks (such as in Thomas's answer, has an @ and at least one . after the @), you should probably just send an email verification letter to the address, and wait for the user to follow a link embedded in the message to confirm that the email was valid.
