Preventing users from registering with xxx@yyy format email address
There is a core function named: function valid_email_address that will check that the syntax is valid.

Example of use:

$mail = $form_state['values']['submitted_tree']['email_address'];
if (!valid_email_address($mail)) {
  form_set_error('[submitted][email_address]', t('The email address appears to be invalid.'));
}