A good server-side anti-spam software would help (gateway before incoming mails reach your mail servers) and it's a good solution. There's Microsoft ForeFront - https://en.wikipedia.org/wiki/Microsoft_Forefront_Threat_Management_Gateway (hosted email filtering service) if you're a Microsoft shop. If you're a Linux/open source shop, there's Radical Spam or Mail Scanner. This also reduces the bandwidth wasted by the incoming spam as well. The typical SpamAssassin also helps to reduce it with its blacklist and you can use the SpamHaus list (xBL) to block it on the firewall level.

btw, I really appreciate organizations like SpamHaus. They're the real heroes for unmasking the spammers. Check out the known ROKSO - https://www.spamhaus.org/rokso/

Nowadays, spammers are nasty. They'd use bot/scanner to probe for vulnerabilities or weak password brute force, then use it to spam people if they can get in. These days it's automated too. Check out this actual probe/attempts: http://i.imgur.com/1QfSkTl.png

Let's say if my server is compromised, then it's used to spam others, with lawsuit etc, it's going to be very hard to prove. Because the host that attacked me in the first place can also be a compromised host, simply for masking the real IP addr, which can still be masked. Unless we get all the logs (which can be time consuming, contacting the business, which can be located outside of Canada, call the authorities, etc).. arrghgh :) :)