How can I use command line formail to change From and To headers

I am working with the output from cron.  The From: and To: headers are like:

From: "(Cron Daemon)" <rgm>
To: rgm

Where the string rgm is from the cron env MAILTO, which is unavailable 
outside of cron (typically it is set to USER).
What I want to change them to is:

From: "(Cron Daemon)" <rgm@$HOSTNAME>
To: rgm@$HOSTNAME

I tried the -R option, but I am not using it right, as it does not seem 
to change anything:
formail -R $USER $USER@$HOSTNAME  >> mailfile < cronout

Is this possible and how?

thanks
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