Mail Sending problem in .Net

I have developed an application with .Net platform. My question is: when I attempt to send an activation password to my customer, I got the following the error.

A connection attempt failed because the connected party did not properly respond after a period of time开发者_如何转开发, or established connection failed because connected host has failed to respond xxx.xxx.xxx.xxx:25

Here is my code:

<system.net>
<mailSettings>
  <smtp from="username@server.com">
    <network host="smtp.server.com" password="xxx" userName="JohnSmith" />
  </smtp>
</mailSettings>
</system.net>

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Are you sure that smtp is active and running on that IP? I'm not able to telnet into it on port 25