Another useful rule set which was posted on the SA mailling list. This once counts the number of recipients and if greater than a threshold add a score
Code:
describe TO_TOO_MANY To: too many recipients
header TO_TOO_MANY To =~ /(?:,[^,]{1,80}){30}/
score TO_TOO_MANY 0.3
describe TO_WAY_TOO_MANY To: way too many recipients
header TO_WAY_TOO_MANY To =~ /(?:,[^,]{1,80}){50}/
score TO_WAY_TOO_MANY 0.3