Unable to read data from the transport connection: net_io_connectionclosed
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)