[imp] Strange extra message: DON'T DELETE THIS MESSAGE -- FOLDER INTERN AL DATA

[Paolo Cavicchini]

Hello, 
I'm experiencing a strange problem...
When I download, with my POP3 client, the messages I received in my inbox,
if I've before opened my inbox with IMP I receive an extra message:

--- Start of the message ---
From: Mail System Internal Data [mailto:MAILER-DAEMON@mbox]
Sent: mercoledì 11 dicembre 2002 10.35
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.
--- End of the message ---

No extra messages if I don't open my inbox with IMP.

My mail system software is sendmail.
Could anyone give me any suggestion?

Thank you
Paolo