Question

I’m behind a proxy and can’t connect with TcpClient to GMail’s POP3. I get the following error:

System.Net.Sockets.SocketException (0x80004005): No such host is known

Any clues?

Code:

var tcpClient = new TcpClient();
try
{
    tcpClient.ReceiveTimeout = 60000;
    tcpClient.SendTimeout = 60000;
    tcpClient.Connect("pop.gmail.com", 995);
    output.AppendLine("Connection OK!");
}
catch (SocketException e)
{
    output.AppendLine(e.ToString());
}
finally
{
    tcpClient.Close();
}

app.config (proxy is set up in Internet Explorer):

<?xml version="1.0"?>
<configuration>
  <system.net>
    <defaultProxy>
      <proxy usesystemdefault="True" />
    </defaultProxy>
  </system.net>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
  </startup>
</configuration>

Answer 1

Nothing. Not possible. It looks like:

• Your company DNS does not include public DNS information.
• So your comptuers ask a proxy which asks an outside DNS.
• This heavily implies you also have no possible routing
• Without Routing no TCP Connection is possible anyway. By design.

Possible workarounds:

• Use a SOCKS proxy. Unlikely to exist.
• Ask your IT Department – the proper way to solve this inot to bypass the firwewall, but to send the emails using the proper channel (internal SMTP Service).
• If the proper way is using gmail, some not too smart person (i.e. a manager) put you into a corner. Then they have to open a TCP connection possibiltiy for you and make the public DNS information available for your computer.

At the end, you can have it both ways – either you force all clients through a proxy, or you have outgiong direct TCP connections.