The native Berkeley mailbox does not have certain features that IMAP or POP access require. So if you access your Berkeley mailbox from an IMAP or POP client, the PMDF IMAP or POP server will create a pseudo-message at the start of your folder to contain the additional details that the Berkeley mailbox itself doesn't have. This pseudo-message by default says "Do not delete this message", (though your system administrators may have modified the text for your site). You should not delete this message; deleting it while you have another IMAP or POP session open to your mailbox may mean that that other session cannot function properly; in any case, the message will be recreated next time you use IMAP or POP to access your mailbox.