We’ve got some weird errors on our Exchange 2007 Mailbox Server. We received this error :

Event ID : 9823
Source : MSExchangeIS Mailbox Store
Log : Application Log
An invalid event history watermark has been detected by background cleanup. The watermark will be deleted.
Database: ‘First Storage Group\Mailboxstore1′.
Consumer: ‘df4b5565-53e9-4776-a824-185f22fb3ca6′.
Mailbox: ‘4ec8c6f9-dbab-43db-a8ac-cb6f62fc556c’.
Watermark counter: ‘0×4AC2AD’.

Funny enough there is no information about this event id.

But I found an explanation :

This Warning event indicates that an invalid event history watermark will be deleted so that all watermark fields are valid. A watermark is a checkpoint stored on the Exchange server that indicates the latest event (an operation such as MailboxCreated, MailboxMoveStarted, or MailboxMoveFailed) was processed on a mailbox. A watermark list helps the event consumers maintain a record of the events processed on a particular mailbox until any given point of time.

A background cleanup process helps delete the old events periodically.

User Action
No user action is required.

Note  If you are not already doing so, consider running the tools that Microsoft Exchange offers to help administrators analyze and troubleshoot their Exchange environment. These tools can help you make sure that your configuration is in line with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. Go to the Toolbox node of the Exchange Management Console to run these tools now. For more information about these tools, see Toolbox in the Exchange Server 2007 Help.