Assuming you have you own mail server, then what you should do is block all outgoing "port 25" packets at the firewall except from your mail server. This is (or should be) standard practice for all networks. The reason for this is if any of your systems get infected with a spam virus, then even if they start spewing spam, it won't reach the outside world, and you won't end up with your domain on all the spam blacklists. (I'm speaking from bitter experience here).
As well, you can block all SMTP packets coming in from outside except those destined for your mail server.
You may also want to screw down other packets - for example, if you have everyone on the network going through a proxy server for internet access (to implement web filtering, for example), then use the firewall to block outgoing http packets from all sources except the proxy server.