Since I made procmail my system mailer, I (and the other people on my
system) got this mail message:
---------- Forwarded message ----------
Date: Thu, 6 Mar 1997 16:34:57 -0500 (EST)
From: Mail System Internal Data
<MAILER-DAEMON(_at_)city(_dot_)ci(_dot_)worcester(_dot_)ma(_dot_)us>
Subject: DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA
This text is part of the internal format of your mail folder, and is not
a real message. It is created automatically by the mail system software.
If deleted, important folder data will be lost, and it will be re-created
with the data reset to initial values.
I'll bet you started using the University of Washington IMAP/POP servers
at the same time. That's what creates this message. See
IMAPSRC/src/osdep/unix/pseudo.c
and
IMAPSRC/src/osdep/unix/bezerk.c
for more info (assuming you are using UNIX with Berkeley mail mailboxes).