If you have not known SpamAssassin yet
As it name might already say, it deals with spam (unwanted emails).
It can (and actually does quite often without your knowledge) run directly on your mail server. Which has the big advantage that the spam mails can be eliminated already on your server and you do not have to download it to your computer.
However, it is very flexible. e.g. you might also want to receive spam, which SpamAssassin has marked as such and move it manually to your trash can.
More about SpamAssassin at its homepage.