Okay...I've calmed down now.
 
Upon thinking more clearly, what we need (or more accurately, what I
think *I* need) is to make it so that all mail from MAILER* (using
the From: field in the headers) should be transferred to Postmaster.
 
If someone gets mail from MAILER*, then it's usually a rejection
notice of some sort.  This, in fact, would fix all my problems.
 
And besides, it's a little easier than checking the Subject line.
 
--Marc
 
P.S.  Afterthought, this wouldn't help nodes who use BITMAIL@xxxx or
other nodes that don't use MAILER@xxxx as their mailer.  But we can
ponder this.