PRW.NET FAQ's : SPAM Control :
How will the mail server identify SPAM messages ?
Message Analysis - This identifies as SPAM all email messages with suspicious headers, including messages with forged or no originating IP address, invalid domains on sender's address, etc. On these messages, the server adds a line to the message header starting with "X-RBL-Warning". In some cases it can also add the text "[Spam Alert - FAILEDTEST]" to the message subject where FAILEDTEST indicates the name of the SPAM test failed.
List Lookup - This identifies as SPAM all messages coming from servers that are on the Open Relay DataBase (ORDB) lists because of their allowing mass mailings from SPAM sources. These messages are rejected by the mail server.
What is SPAM ? Why should it be controlled ?