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Yellow lists are lists of hosts that send a mix of spam and non-spam. Email services like Yahoo, Gmail, and Hotmail are examples of mixed source servers. These servers should never be either black or white listed. These are servers where the IP address yields no useful information as to whether the message is spam or ham.

Multi-Color List Processing Logic

When looking at lists from a black/white/yellow perspective there is an order to the list processing. First the yellow lists are tested. If the message is yellow listed then checking black and white lists isn't necessary. The the white lists are checked. If the message is white listed it can be passed without having to run it through SA or SA can short circuit the tests and declare the message ham. Then the black lists are checked and if it is in a trusted blacklist or several less trusted blacklists the message can be rejected.