You can change the default header of your mail messages. That's a handy way to put in changes that you want made to the headers of all (or most) of the messages you send.
The first time you click the Compose Message button, xmh runs the MH comp command and reads a copy of the header that comp makes. (It actually runs comp -file tempfile -nowhatnowproc -nodraftfolder and gets a copy of tempfile.) It does the same kind of thing for the Reply and Forward commands.
By default, those MH commands read the template draft header files named components, replcomps, and forwcomps. To change the xmh draft header, you can change those template draft files.
For instance, if you make a components file like this:
To:
Reply-to: ehuser@asun.xxx.yyy.zzz
cc:
Fcc: outbox
Subject:
---------
that's what the default fields in your composition window will
look like when you use Compose Message.
Note that xmh only runs comp once per xmh session and saves the result. Therefore, if you change your components file, you'll need to restart xmh for the changes to take effect there. But xmh has to run the MH repl and forw commands before each message that you reply to or forward. Your changes to replcomps and forwcomps take effect right away.