Happety, it's a catch-all.
Obviously, I don't know your situation and how your mail has been handled previously, so this is general based on how I personally have mine setup.
I mentioned my "system" in the first post of this thread.
I am a huge fan of e-mail aliases and a huge, what's the word, unfan of catch-all addresses. Knowing your domain name is easy, then a spammer simply enters whatever they want in front and it's mail you're going to get. Aliases, though not always the easiest to manage, offer complete control over e-mail.
This is easier said than done after the fact as you probably have tons of addresses already floating around that you want to keep. I would find the addresses being used that you want to keep and make aliases out of them, also setting up global spam assasin settings, then shutting off catch-all.
The somewhat new command line e-mail setup has made things so much easier. As soon as I need a new alias, I simply telnet into my account, which I can easily do from any computer or my phone. Type in vaddalias newalias email@example.com
and done! 10 seconds. Start getting spam on one of them, same thing, vdelalias and no more spam.