🚀 go-pugleaf

Sendmail makes it relatively easy to build such mail interfaces.
It has an option (-t) which says, "Read the header and message from
standard input and decide who to send it to".  This lets mh, vnews,
etc. avoid mesing with headers and just let them pass the whole message
to sendmail.

I believe it deals with the "authentication" issue by inserting a Sender:
field if the supplied From: field doesn't match the one it would have
provided (except for commentary).  It probably doesn't deal with
duplicated fields (eg an existing Sender:, or some Received: lines).
It removes all Bcc: lines from the header after building the envelope.

There was a message from someone at IBM suggesting that you should be
able to edit a version of the message with groups expanded, then 
for sending it should compress them out again?  Why would you want to?