Can't Send Smtp Email From Network Using C# Error / Unable To Connect To The Remote Server

I have my code here, it works fine from my home, where my user is administrator, and I am connected to internet via a cable network.

But, problem is when I try this code from my work place, it does not work. Shows error:

"unable to connect to the remote server"

From a different machine in the same network:

"A socket operation was attempted to an unreachable network 209.xxx.xx.52:25"

I checked with our network admin, and he assured me that all the mail ports are open [25,110, and other ports for gmail].

Then, I logged in with administrative privilege, there was a little improvement, it did not show any error, but the actual email was never received.

note that, the code was tested from development environment, visual studio 2005 and 2008.

try
{
MailMessage mail_message = new MailMessage("xxxxx@y7mail.com", txtToEmail.Text, txtSubject.Text, txtBody.Text);
SmtpClient mail_client = new SmtpClient("SMTP.y7mail.com");
NetworkCredential Authentic = new NetworkCredential("xxxxx@y7mail.com", "xxxxx");
mail_client.UseDefaultCredentials = true;......