Open the server settings editor, go to [SMTP] tab and check:
1. That in your server settings the SMTP protocol is activated 2. That Accept/Deny addresses
If (1) and (2) are ok then from your machine try to telnet to 25. If the connection fails ask your network guys if there is an access list that prevents access to the port 25 of your service desk application server.
Have you tried restarting your apps server ? It could be running low on resources. Also try pinging your apps server from a remote client and check how long it takes. (could be network issues, your nic..etc)