Forged X-Spam headers

[Christopher Martin]

I have been noticing the occasional spam slipping past spam assassin 
unscathed lately but have been a bit busy to pay attention (one spam a 
day is much better than the 150 each user used to get). I paid a bit 
more attention to one the other day and noticed it had an X-Spam header 
before it got to spam assassin. For a few seconds I thought that maybe 
my ISP had started tagging silently, until I noticed that the spam score 
was -83... Not the positive score it should have been, so I deduced that 
spammers are forging the X-Spam header to slip by the classification rules.

I had a search on the Nabble archive for this list and couldn't find 
anything specifically about this (it probably got lost in the million 
results that just about any search phrase produces!) so I am hoping 
someone can point me at a solution if it's been discussed before.

Is there an option for the spamassass-milter to strip X-Spam headers 
before the mails are handed to Spam Assassin for processing? If not, is 
there another milter I will need to use? I guess I can put it in between 
milter-regex and spamass-milter.

Any ideas?

Chris M