The easiest way to check that the outbound connection isn't being blocked on port 25 TCP is using telnet.
Microsoft provides information on how to do that here
If you can connect to a remote mail server on port 25 TCP outbound, the problem is most likely with the local configuration of the Exchange server.
If you can't
make a telnet connection on port 25 TCP to a remote mail server then it may be filtering at the ISP level, on your router's firewall, on the Exchange computer's Windows firewall or any installed antivirus software. Confirm that port 25 TCP outbound isn't possible using a different computer to see whether it is an issue with just the computer running Exchange.
Once you've located where the problem is, you can take the next step to narrowing the cause down or identifying it conclusively.