[Spambayes] help! can't read the emails once trained!

[Eli Spizzichino]

Hello,
I run sb_mboxtrain.py and it added X-Spambayes-Trained: ham/spam header 
to all the emails processed but now thunderbird or squirellmail can't 
read them anymore with such a header! I'm using courier-imap-4.0.6-r2.
First how can I revert the situation?  I need to do so asap.
Then I'd like to understand what's went wrong?

thanks
Eli