PHP5 comes with a fantastic little filter library:
CODE
if(filter_var($email, FILTER_VALIDATE_EMAIL))
{
// Email format is correct
}
If you're not on Zymic hosting you can extend the check to start a smtp session with the domain's mail exchange servers and check the email address actually exists. This method isn't full proof if someone sets their MX records to point to a mailserver which has been setup as a relay.
Email verification is the best method to ensure the mailbox actually exists.