PRW.NET FAQ's : SPAM Control :
How can my email program block SPAM ?
First, you need to decide whether to delete all SPAM messages, bounce them to the sender or move them to a different folder. Until you test your rules for an extended period of time, we suggest that you do not delete these messages since a small percentage of them may be valid. Bouncing may not be a good idea since some messages may have invalid sender addresses and boncing could lead to mail loops. The more conservative approach is to create a "SPAM" folder, redirect your messages to it and periodically delete unwanted messages from it.
These are example Outlook rules for redirecting SPAM to a folder:
Spam - Message Analysis
Apply this rule after the message arrives
with X-RBL-Warning in the message header
move it to the SPAM folder
and stop processing more rules
Spam - List Lookup
Apply this rule after the message arrives
with SpamAlert in the subject
move it to the SPAM folder
and stop processing more rules
How will the mail server identify SPAM messages ?