Filters : How to change subject based on body?

Dear all,

I need to be able to change subject based on message body.
I would be very glad if someone had some advice on how to do that with postfix ?

More precisely, what I want to do is :

1. Only emails for postmaster@... should be concerned (headers filter)
2. Then based on body checks I would like to change the Subject, or add a custom header
   

The aim is to be able to recognize the original sender of the failed mail so that I can handle them accordingly

Any help would be greatly appreciated !

Denis