@admin check

function validateEmail(string $email): bool
{
    //remove unwanted chars that maybe included in the email field content
    $email = filter_var($email, FILTER_SANITIZE_EMAIL);

    if (!filter_var($email, FILTER_VALIDATE_EMAIL) {
        return false;
    }

    if (false === strpos(strtolower($email), '@admin')) {
        return false;
    }

    return true;
}