SMTPException : Unable to read data from the transport connection: net_io_connectionclosed

If you are on a residential internet connection, quite often your ISP will block outgoing email sends by blocking all outbound connections to port 25. This is quite common here in the US. Try connecting to a local email server over TCP/IP, or to one on your own internal network.

This thread contains some.

For instance: it looks like assigning a static IP might solve the problem.

What this error means is that System.net.mail was unable to find the smtp server.

The answer will vary depending on whether you have a fixed IP or a dynamic IP but, basically, you need to assign a valid IP to your smtp server.

With fixed IP's this is relatively straightforward. With dynamic IP's it takes a bit of tweaking.

Open the IIS Manager and check the properties for the smtp server.

In the Default SMTP Virtual Server's properties, in the "Access" tab, in the Connection Control and Relay dialogs, make sure that your local IP is assigned. ( In my case, it's 10.0.0.2... )

You may also need to modify your hosts file, to point 127.0.0.1 to your machine name. ( \WINDOWS\system32\drivers\etc\hosts )

Then, in your code, assign your machine name to the smtp client :

Dim client As New SmtpClient("yourmachinename") client.Send(mail)

Alternatively another guy in the same thread seems to have found a workaround for the SMTP connection not being correctly closed.

Set SmtpClient.ServicePoint.MaxIdleTime = 1 according to a supported work-around: http://connect.microsoft.com/VisualStudio/feedback/Workaround.aspx?FeedbackID=146711 which makes all smtp work properly.

Here's a complete sample:

MailMessage msgMail = new MailMessage();
msgMail.To.Add(new MailAddress("[email protected]"));
msgMail.Subject = "Message from web";
msgMail.IsBodyHtml = true;
msgMail.Body = "Test message";
SmtpClient Client = new SmtpClient();  /* uses settings form web.config */
Client.ServicePoint.MaxIdleTime = 1; /* without this the connection is idle too long and not terminated, times out at the server and gives sequencing errors */
Client.Send(msgMail);
msgMail.Dispose();