As I was checking the spam filters on the server this morning, I saw a wave of crap from bogus earthlink accounts. The main reason was that a website (yourdomainname.com) was set to send ALL email addressed to that domain to their earthlink email. (Default Address)
This is not a good setup, as you then get hit by dictionary attacks where they send to every name and word @yourdomainname.com, AND it gets delivered.
A better setup is to decide what addresses you are actually going to expect mail to, like 'info@', 'sales@', 'BigDaddy@', and set those up as Individual forwards.
Then set Default Address to: :fail: (yes, include the colons).
This will tell the server to not accept any mail except for 'known' addresses (your forwards, or if you setup a mail account).
You'll see a huge decrease in spam.