SpamAssassin Rule: URIBL_GREY
Standard description: Contains an URL listed in the URIBL greylist
"grey.uribl.com - This lists contains domains found in UBE/UCE, and probably honour opt-out requests. This list can and probably will cause False Positives depending on your definition of SPAM. This zone rebuilds several times a day as necessary."
The default scores for this rule can be found in the online list of tests.