SmtpCommandExceptionErrorCode Property

public SmtpErrorCode ErrorCode { get; }

public static void SendMessage (MimeMessage message)
{
    using (var client = new SmtpClient ()) {
        try {
            client.Connect ("smtp.gmail.com", 465, SecureSocketOptions.SslOnConnect);
        } catch (SmtpCommandException ex) {
            Console.WriteLine ("Error trying to connect: {0}", ex.Message);
            Console.WriteLine ("\tStatusCode: {0}", ex.StatusCode);
            return;
        } catch (SmtpProtocolException ex) {
            Console.WriteLine ("Protocol error while trying to connect: {0}", ex.Message);
            return;
        }

        // Note: Not all SMTP servers support authentication, but GMail does.
        if (client.Capabilities.HasFlag (SmtpCapabilities.Authentication)) {
            try {
                client.Authenticate ("username", "password");
            } catch (AuthenticationException ex) {
                Console.WriteLine ("Invalid user name or password.");
                return;
            } catch (SmtpCommandException ex) {
                Console.WriteLine ("Error trying to authenticate: {0}", ex.Message);
                Console.WriteLine ("\tStatusCode: {0}", ex.StatusCode);
                return;
            } catch (SmtpProtocolException ex) {
                Console.WriteLine ("Protocol error while trying to authenticate: {0}", ex.Message);
                return;
            }
        }

        try {
            client.Send (message);
        } catch (SmtpCommandException ex) {
            Console.WriteLine ("Error sending message: {0}", ex.Message);
            Console.WriteLine ("\tStatusCode: {0}", ex.StatusCode);

            switch (ex.ErrorCode) {
            case SmtpErrorCode.RecipientNotAccepted:
                Console.WriteLine ("\tRecipient not accepted: {0}", ex.Mailbox);
                break;
            case SmtpErrorCode.SenderNotAccepted:
                Console.WriteLine ("\tSender not accepted: {0}", ex.Mailbox);
                break;
            case SmtpErrorCode.MessageNotAccepted:
                Console.WriteLine ("\tMessage not accepted.");
                break;
            }
        } catch (SmtpProtocolException ex) {
            Console.WriteLine ("Protocol error while sending message: {0}", ex.Message);
        }

        client.Disconnect (true);
    }
}