AIX mbox file

Hi,
on AIX 7.1

Does anyone know mbox file ? What is it for? Should it have a fixed size? If become large how to resize?
The current content is:

From MAILER-DAEMON Fri Feb 21 13:19:27 2020
Date: 21 Feb 2020 13:19:27 +0100
From: Mail System Internal Data <MAILER-DAEMON@server>
Subject: DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA
Message-ID: <1582287567@server>
X-IMAP: 1582082091 0000000097
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.

Thanks.

??
Every user has a message box... If its growing its because the concerned user is not bothered in looking what he as received...
When I see such case I reset to 0...
Mostly used when you have batches running under your UID (cron or at...) and messages such as you had files open when shutdown or reboot etc...

Use e.g. the mail / mailx command to read / delete the mails your user received.