A score factor. When the first item matches, the score of the message is increased or decreased by this value. A time limiting factor. B.C. A type factor. This specifies the way the first factor matches.
Available types depend on keys.
R , S , E , and F are case sensitive. This key matches From header, and scores all follow-ups to the message. For example, it would be useful for increasing scores for follow-ups to you own article. You can use the same types as From except for f . You can make sure the whole thread including messages that does not have all ancestors Message-ID in References is scored.