Why can SmtpClient.SendAsync only be called once?

Question

I\'m trying to write a notification service (for completely legit non-spam purposes) in .NET using SmtpClient. Initially I just looped through each message and sent it, however

Answer 1

There is a reason to reuse the SmtpClient, it limits the # of connections to the SMTP server. I cannot instantiate a new class SmtpClient class for each thread the reports are building on or the SMTP server will balk with too many connections error. This is the solution I came up with when I couldn't find an answer here.

I ended up using an AutoResetEvent for keeping everything in sync. That way, I can keep calling my SendAsync in each thread, but wait for it to process the email and use the SendComplete event to reset it so the next one can continue.

I setup the Auto Reset Event.

        AutoResetEvent _autoResetEvent = new AutoResetEvent(true);

I setup the shared SMTP Client when my class is instantiated.

        _smtpServer = new SmtpClient(_mailServer);
        _smtpServer.Port = Convert.ToInt32(_mailPort);
        _smtpServer.UseDefaultCredentials = false;
        _smtpServer.Credentials = new System.Net.NetworkCredential(_mailUser, _mailPassword);
        _smtpServer.EnableSsl = true;
        _smtpServer.SendCompleted += SmtpServer_SendCompleted;

Then when I call the send async, I wait for the event to clear, then send the next one.

        _autoResetEvent.WaitOne();
        _smtpServer.SendAsync(mail, mail);
        mailWaiting++;

I use the SMTPClient SendComplete event to reset the AutoResetEvent so the next email will send.

private static void SmtpServer_SendCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
        {
            MailMessage thisMesage = (MailMessage) e.UserState;
            if (e.Error != null)
            {
                if (e.Error.InnerException != null)
                {
                    writeMessage("ERROR: Sending Mail: " + thisMesage.Subject + " Msg: "
                                 + e.Error.Message + e.Error.InnerException.Message);
                }

                else
                {
                    writeMessage("ERROR: Sending Mail: " + thisMesage.Subject + " Msg: " + e.Error.Message);
                }
            }
            else
            {
                writeMessage("Success:" + thisMesage.Subject + " sent.");
            }
        if (_messagesPerConnection > 20)
        {  /*Limit # of messages per connection, 
            After send then reset the SmtpClient before next thread release*/
            _smtpServer = new SmtpClient(_mailServer);
            _smtpServer.SendCompleted += SmtpServer_SendCompleted;
            _smtpServer.Port = Convert.ToInt32(_mailPort);
            _smtpServer.UseDefaultCredentials = false;
            _smtpServer.Credentials = new NetworkCredential(_mailUser, _mailPassword);
            _smtpServer.EnableSsl = true;
            _messagesPerConnection = 0;
        }
            _autoResetEvent.Set();//Here is the event reset
            mailWaiting--;
        }