If you know the mail server is up and running, check whether you can talk over port 25 to your mail server. This can be done using a one-line command: telnet <appserver name> 25 Substitute the <appserver name> with your own server.
Once you type this into your DOS window and hit return, you should see some sort of response from your mail server, as shown below:
Trying 208.75.196.84... Connected to cu190.cubit.sp.collab.net (208.75.196.84). Escape character is '^]'. 220 cu190.cubit.sp.collab.net SMTP Server (JAMES SMTP Server 2.2.0) ready Mon, 27 Jul 2009 06:38:20 -0700 (PDT)