When I go to the local terminal on my Exchange box (SBS 2008) I can do this:
telnet 127.0.0.1 25 220 Exchange banner here EHLO example.com 250 Server name
However when I go from another box, or from the actual IP of the server I get this:
telnet 192.168.21.20 25 220 Exchange banner here EHLO example.com 421 4.4.1 Connection timed out
Connection to host lost.
The odd thing is, this server is currently in production and working fine (receiving mail for our entire domain). But my C# programs can't send mail to it (they get this same error).