mail.php

<?php
function check_email_address($email) {
        // First, we check that there's one @ symbol, and that the lengths are right
        if (!preg_match("/^[^@]{1,64}@[^@]{1,255}$/", $email)) {
            // Email invalid because wrong number of characters in one section, or wrong number of @ symbols.
            return false;
        }
        // Split it into sections to make life easier
        $email_array = explode("@", $email);
        $local_array = explode(".", $email_array[0]);
        for ($i = 0; $i < sizeof($local_array); $i++) {
            if (!preg_match("/^(([A-Za-z0-9!#$%&'*+\/=?^_`{|}~-][A-Za-z0-9!#$%&'*+\/=?^_`{|}~\.-]{0,63})|(\"[^(\\|\")]{0,62}\"))$/", $local_array[$i])) {
                return false;
            }
        }
        if (!preg_match("/^\[?[0-9\.]+\]?$/", $email_array[1])) { // Check if domain is IP. If not, it should be valid domain name
            $domain_array = explode(".", $email_array[1]);
            if (sizeof($domain_array) < 2) {
                return false; // Not enough parts to domain
            }
            for ($i = 0; $i < sizeof($domain_array); $i++) {
                if (!preg_match("/^(([A-Za-z0-9][A-Za-z0-9-]{0,61}[A-Za-z0-9])|([A-Za-z0-9]+))$/", $domain_array[$i])) {
                    return false;
                }
            }
        }

        return true;
    }

filter_var('bob@example.com', FILTER_VALIDATE_EMAIL) works most of time, but not perfect.