BAD MSG:
wants to specify an address to which "reply to sender" should go,
he From field is the place to do it.

Reply-to be used to specify the sender's preferences about where
replies to this message should go by default, instead of *any*
of the From, To, and Cc addresses.

and that user agents be encouraged to implement:

"reply to sender"	

	send to From address(es)

"reply to sender and recipients"

	send to From, To, and CC addresses

"reply" 	
	send to Reply-to address(es) if the header exists, 
	else treat as either "reply to sender" or 
	"reply to sender and recipients".  (I prefer the 
	latter; many others prefer the former and it's 
	arguably the one that's more widely implemented.)

Keith