🚀 go-pugleaf

You can use the init file mechanism to set up a new menu that adds a
Reply-to header field before sending the mail.  Something like this
should do the trick:

addmenu sendmessage-compound-operation "Send W/Reply-To" sendmessage
sendmessage inherit "sendmessage-add-header Reply-to: nsb@bellcore.com;
sendmessage-send-message"

This belongs in your .messagesinit AND .sendmessageinit files, if you
ever run the program under both the names "messages" and "sendmessage". 
What I do is I have both of those init files include another file called
".amsinit" that has all the lines I want to use in both.