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From: rms@GNU.AI.MIT.EDU (Richard Stallman)
Newsgroups: gnu.emacs.help,comp.emacs
Subject: Appending to RMAIL file
Message-ID: <9104092229.AA02599@apple-gunkies.gnu.ai.mit.edu>
Date: 9 Apr 91 22:29:17 GMT
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It is asking for trouble to append Unix-format messages to an Rmail file.
It will work when the file is not in core, *provided*
you don't add any RMAIL-format message to the file.
If you do, then any Unix-format messages before them won't
be converted.

The safe thing to do is output Unix format messages to an inbox file
and then get new mail into the rmail file from that inbox.
This can be made to happen automatically by adding that inbox file
to the inbox list of the rmail file.