You are running an Arcserve RHA Replication or High Availability scenario on a Exchange 2007 master/replica environment and after starting the scenario you notice Event ID 1020 being written to the logs repeatedly every few minutes. You have at least one Public Folder database on your master server and have recently applied rollup 8 or higher for Exchange 2007 SP1. This behavior is now as intended by Microsoft starting with Exchange 2007 Rollup 8 for SP1 and in general is safe to ignore for XOsoft customers.
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 Arcserve RHA 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 Arcserve RHA is active by design it uses it's own replication technology in place of standard Exchange public folder replication. This means that for most Arcserve RHA 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 Arcserve RHA to disregard the above event when Arcserve RHA 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.