It's not so obvios task. Some may have john@smith.gmail.com email address, others john_smith@gmail.com (or r2d2@gmail.com). Check it with this script:
<?php
if( $_POST['email'] ){
$separator = array('@', '_', '-');
$array = explode('.', str_replace($separator, '.', $_POST['email']));
echo $array[0] . ' - ' . $array[1];
}
?>
<form action="<?php echo $_SERVER['PHP_SELF']?>" method="POST">
Email: <input type="text" name="email" />
<input type="submit" value="check" />
</form>
?>