Extract Reply-To: or From: to file

I've a friend who wants to save just the senders address to a file for later
follow-ups to requests for info.  I figure the recipe should check for a
Reply-To: field and if that is non-existant then grab the From: field. 

After trying various `formail -rtzx...` combinations to grab that info, I
can't figure out how to do anything other than add foo(_at_)bar to my results
file.  Anybody care to point me in the right direction?  Yes, I'm way over
my head.  Be gentle, please.


rick...