I'm getting "System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed." but only occasionally. I'm confused because:

1) The SMTP server is the local machine so why would the connection ever fail? and
2) The error only happens sometimes

There was a similar question posted here: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=418540&SiteID=1 but it seems they were having problems sending any mail to begin with.

Here is the full stack trace:

System.Net.Mail.SmtpException: Failure sending mail. > System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed.
at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine)
at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)
at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)
at System.Net.Mail.SmtpReplyReader.ReadLine()
at System.Net.Mail.CheckCommand.Send(SmtpConnection conn, String& response)
at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
at System.Net.ClosableStream.Close()
at System.Net.Mail.MailWriter.Close()
at System.Net.Mail.SmtpClient.Send(MailMessage message)

I'm unable to recreate this error since it only happens every couple of days or even weeks.

Has anyone had a similar error and been able to resolve it?

Thanks.