Is there something in the log file?
Should be in "ARSystem/AREmail/logs/stderr.log"
First, can you ping your stmp server from your ARS server ?
If yes, next can you do a telnet <stmpserver> 25 from your ARS server ?
In the first case, you probably have a problem of name resolution (normal if your DC controler has changed)
In second case, verify the ip address of your ARS server is the same as before (it should) and verify with your smtp administrator if this IP is always allowed to use the port 25.