bogus message in mbox

[Jorge Almeida]

I noticed that mboxes start with something like this:
 	From MAILER-DAEMON Sat Oct 14 23:10:03 2006
 	Date: 14 Oct 2006 23:10:03 +0100
 	From: Mail System Internal Data <MAILER-DAEMON at math.ist.utl.pt>
 	Subject: DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA
 	Message-ID: <1160863803 at math.ist.utl.pt>
 	X-IMAP: 1157659905 0000004736 NonJunk
 	Status: RO

 	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.


This message doesn't appear to be visible by the MUA, as intended, I
suppose. The problem is that I need to split the mbox into individual
messages (via safecat) in order to do further processing, and I don't
want to end up with a bogus message. 
Anyone knows what causes this? The explanation "by the mail system
software" doesn't say that much. The MTA is sendmail. Is it the culprit?
Do other MTAs do the same?

-- 
Jorge Almeida