should the spam score increase

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Hi all,

I recently implemented a rule kindly provided by John Hardin - 
MIME_IMAGE_ONLY
I watched it carefully and it only caught spam YAY
I had to up the score as they were all still getting through due to 
bayes_00 (score -3)
Having done that it is all now getting blocked which is great but my 
question is should they system now be 'learning' these and thus changing 
the bayes_00 to bayes_50 etc

If not, what is the best method to go about 'learning' these spam.

Thanks
Kate