Spam filtering on the server can be done in two different ways.
1. Greylisting and black lists work at the instant that some remote host connects to our server to try to deliver mail. If such mail appears to be spam, it is rejected by greylisting and black lists before it ever enters the system. Thus, such mail is never delivered to you. In very rare cases, if legitimate mail is incorrectly rejected, the sender will see an error message and he will know that the message never reached you. Thus, greylisting and black lists will never cause mail to be silenly lost.
2. SpamAssassin operates on mail that has already been accepted by the server. If SpamAssassin decides that a message should be classified as spam, it will deliver the message in accordance with your instructions. The message may be silently deleted, or marked as spam and delivered into your inbox, or delivered into a spam folder. In a rare case, if a legitimate message is classified as spam, the sender will not know that his mail to you appeared to be spam. If you do not see this legitimate message (perhaps because SpamAssassin silently deleted it, or because it ended up in your spam folder), the message is silently lost and the sender will not know this.
To minimize the risk of legitimate mail that is silently lost, you should be sure to allow greylisting and/or black lists to operate as a first-level defense against spam. You can then use SpamAssassin as a second-level defense against any spam that gets past the first-level defense.
Following sections of this FAQ will show you how to configure all types of spam-fighting mechanisms mentioned above.
This document is: http://faq.rahul.net/cgi-bin/fom.cgi?file=8
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