public bool TestEmailRegex(string emailAddress)
{
    string patternStrict = @"^(([^<>()[\]\\.,;:\s@\""]+"
        + @"(\.[^<>()[\]\\.,;:\s@\""]+)*)|(\"".+\""))@"
        + @"((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"
        + @"\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+"
        + @"[a-zA-Z]{2,}))$";
    Regex reStrict = new Regex(patternStrict);
    bool isStrictMatch = reStrict.IsMatch(emailAddress);
    return isStrictMatch;
}

Исходник