What are the drawbacks of relying solely on syntax validation for email addresses in PHP?

Relying solely on syntax validation for email addresses in PHP can lead to false positives or false negatives. It may not catch all invalid email addresses, such as those with unconventional characters or missing domain extensions. To improve accuracy, it's recommended to also perform domain validation by checking the DNS records for the email address.

function validateEmail($email) {
    if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
        list($user, $domain) = explode('@', $email);
        return checkdnsrr($domain, 'MX');
    }
    return false;
}

$email = "example@example.com";
if (validateEmail($email)) {
    echo "Email address is valid.";
} else {
    echo "Email address is invalid.";
}