Mailbox Over High Mark

I received the below message in my Microsoft Outlook inbox from "admin" and need further explanation as to what generated it and is it referring to my Outlook personal folders inbox. Also, what does it mean by "high water mark".  I am using Windows XP Professional and Microsoft Office Outlook 2007. Please advise.

"Your mailbox is over the high water mark.
Please delete some messages from your mailbox!"
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How many mails do U have in ur mailbox ?
Nick RhodeIT DirectorCommented:
That is usually from your ISP meaning at their location it is getting full.  Usually due to "leave a copy on the server" being checked.  Log into your ISP account and check the mail in there or you would have to remove some to make more room.

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