You start to see the event ID error 1020 from the MS Exchange Store Driver source in your windows event logs after upgrading to rollup 8 or beyond for Exchange 2007.
The error event 1020 was added to Exchange 2007 with Rollup 8 for Service Pack 1, and is described in the following Microsoft KB article:
'958239 Exchange Server 2007 does not generate an event log message for public folder replication messages even though a property validation exception is thrown and the replications do not occur' http://support.microsoft.com/default.aspx?scid=kb;EN-US;958239.
Prior to this update, these replication messages were being dropped silently by the Exchange server. For customers not running XOsoft this became cause for concern because of the difficulty in discovering and troubleshooting this behavior so this event was added with an error level designation.
However when XOsoft is active by design it uses its own replication technology in place of standard Exchange public folder replication. This means that for most XOsoft customers events regarding standard Exchange public folder replication are not cause for concern and solely represent an inconvenience. Error event 1020 is being reported because while the scenario is Active, the active Public Folder store is attempting to send public folder hierarchy messages to the stand by's Public folder stores. Because the stand by's public stores are offline the messages are coming back to the active server, failing property validation, and being dropped, the event is triggered.
As noted above prior to Update Rollup 8 for SP1, these messages were just being dropped silently, so there is no functional difference in behavior other than the event being logged. Since there is no possible way to disable or suppress this event as of the latest available release (Exchange 2007 SP2), Microsoft is recommending to all customers running XOsoft to disregard the above event when XOsoft is active on Exchange 2007 SP1 rollup 8 or later.
For more information regarding this event please see the Microsoft KB article above or contact Microsoft support.