emacs mail client for mailbox files?

[phr-2002]

I seem to remember a while back, someone wrote an Emacs mail client
that had an RMAIL-like interface but worked on ordinary mail files.
I.e., when you read your mail, it operates on your standard inbox, and
does NOT move the incoming mail to a separate rmail file.  Hopefully,
it saves old messages in an mbox file.

Anyone know if that exists and where I can find it?  Thanks.