Spam, a.k.a. UCE, or Unsolicited Commercial E-mail.
Each person's definition of what a constitutes an annoyance is different. Luckily, with SpamAssassin, there's an easy way to deal with spam, whatever your happens to be.
This how-to guide assumes you setup your e-mail account as an IMAP account (POP is not supported at this time) and SpamAssassin is turned on for your domain. If you can't see the referenced folders after the first step, contact your site administrator.
Step 1. Open your e-mail program. Have a look under the name of the target IMAP account. Under the Inbox, you should see three folders named 'Learn Ham', 'Learn Spam' and just plain 'Spam'. If you don't see them, try the following:

Step 2. If you can't see the three folders, contact your admin now. If you can, congrats, you've done the hard part. The three folders are used thusly:
The goal is to get 200 or more e-mails of each type in the 'Learn...' folders. Also, you need to try to have about the same number of e-mails in each folder. Once you have around 200 each, any messages that the computer determines are spam will go directly to the 'Spam' box, bypassing your e-mail Inbox. You need to check this box every once in a while to make sure things are working as expected.
It's important not to put e-mails in the 'Learn...' boxes that you want to keep. Each day, the server will check for e-mail there, update its database and send you an automated message. It will give you a count of how many of each type of 'Learn...' mail have been added to its database. The e-mails that were put in the 'Learn...' boxes will then be deleted.
After you get around 200 e-mails in each, you should notice a substantial reduction in the amount of spam you receive in your Inbox. Contact your admin if something seems amiss.