I agree that when dealing with prexisting folding it is better to preserve it.  The case I was talking about is, say, prepending re to a subject and refolding it.  It is the transformation step where I think turning fws into a single space makes sense.  But I absolutly want this to be controllable and hope that experience can inform the chosen defaults.  I also expect to be getting feedback from the mailman folks well before anything gets set in stone.