/// <remarks>No special characters Space and "(),:;<>@[\] are allowed, though valid by standard.</remarks>
        public static bool IsValidEmail(string email)
        {
            if (string.IsNullOrWhiteSpace(email)) return false;
            string LocalName = "";
            int state = 0;
            foreach(var c in email)
                switch (state) {
                case 0:// wait for the first valid character of mailbox
                    if (!IsValidEmailChar(c)) return false;
                    LocalName += c;
                    state = 1;
                    break;
                case 1:// accumulate mailbox string or jump to domain by @
                    if (c == '@') {
                        // validate mailbox on special case of "."
                        if (LocalName.StartsWith(".") || LocalName.EndsWith(".") || LocalName.Contains("..")) return false;

                        state = 2;
                        break;
                    }
                    if (!IsValidEmailChar(c)) return false;
                    LocalName += c;
                    break;
                case 2:// wait for any valid DNS character
                    if (!IsValidDNSChar(c)) return false;
                    state = 3;
                    break;
                case 3:
                    if (c == '.')
                        state = 2;
                    else
                        if (!IsValidDNSChar(c)) return false;
                    break;
                }
            return state == 3;
        }