Mac OS X has its own spam filter which you can train. In the Mail > Junk Mail options, you can switch into 'Training' mode, where you see recognised spam in yourd Inbox. We would recommend leaving 'Trust junk mail headers set by your ISP' ticked, and also exempting senders in your address book and anyone using your full name.
You should then correct the decisions made by Mail. If you see something incorrectly marked as spam, open it and click on 'not junk'. If something should be marked, go to the 'Message' menu and 'mark as junk'. Once it seems to work OK turn the system from 'Training' to 'Automatic' mode.
You can also set up custom filters to take advantage of GreenNet's invisible header markings (and in later versions of Mail, even mark filtered messages as junk, which may or may not be worthwhile).
1. Go to Mail > Preferences > Rules.
2. Click 'Add Rule'
3. Under 'If' choose 'Edit Header List'
4. Click '+' to add a new header, and add 'X-Spam-Level', and click OK.
5. Type a name for the rule, e.g. 'SpamAssassin' and select 'If all the conditions are met'
6. Choose 'X-Spam-Level' 'contains' '*****'.
6a. You may want to add the condition 'Sender is not in my address book'
7. Choose to move the messages to the 'Spam' or 'Junk' mailbox.
8. Test the system works
See http://www.uab.edu/it/email/spam-filter-rules/applemail.html for similar instructions.
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