error while sending mail. : Failure sending mail

open a command prompt (I know, very old-fashioned, but it does the trick :-) and type:
telnet mailsvr 25

mailsvr is ofcourse the name of your mailserver. Telnet needs to be enabled in programs and features if you use Windows 7 (it is turned off by default). The cmd window should refresh and show a message stating you are connected to the mail server. If it does not, there is some network issue with the SMTP protocol (which is port 25). If you get a connection you can even try to send email using the SMTP commands in the window. How the SMTP commands work depends on the email server you are using.

System.Net.Mail reads SMTP configuration data out of the standard .NET configuration system (so for ASP.NET applications you’d configure this in your application’s web.config file). Here is an example of how to configure it:

<pre lang="xml"><system.net>
    <mailSettings>
      <smtp from="test@foo.com">
        <network host="smtpserver1" port="25" userName="username" password="secret" defaultCredentials="true" />
      </smtp>
    </mailSettings>
  </system.net>

Hope this Helps out.