If you keep getting "Request processing continued, stage CreatingInitialSyncCheckpoint" and "Transient error MapiExceptionNotFound...
has occurred" messages during your Exchange mailbox migration, a good place to start is to view the migration reports -- they will show more detail about each mailbox.
From the Exchange Server 2013 admin center, navigate to "Recipients" and select "Migration." The list of all current batches will be displayed. Select the batch with the failed moves and either click on Edit or double click on the batch. Select "Migration reports" and a list of reports will pop up.
Alternatively, if you have a large batch of mailbox moves, you could use the Exchange Management Shell. The Get-MoveRequest -MoveStatus Failed | Get-MoveRequestStatistics –includereport cmdlet is an effective way to pull the reports for only the failed move requests. There is also a –Diagnostic parameter to use with Get-MoveRequestStatistics to provide more details for troubleshooting.
If the batch has a prolonged state of Syncing, click on "View details" for the batch in the Exchange admin center's migration section to see the current state of each Exchange mailbox migration. Errors are shown here even before the reports are created. You can also look at the application log in "Event Viewer" for logs from the Mailbox Replication Service.
The details provided here shed some insight. While the CreatingInitialSyncCheckpoint error message helps tell us at what stage the issue is occurring, the MapiException error is most helpful.
There are a variety of possible MapiExpection errors like MapiExceptionNetworkError, MapiExceptionLogonFailed and MapiExceptionJetErrorPageNotInitialized. In this case, the error is MapiExceptionNotFound. This is usually followed by an additional description like "Unable to copy to target", "Unable to open message store" or "The system will retry (#/#)." This clarification also helps narrow down the root cause of the Exchange mailbox migration issue.
Since I do not have access to this particular log, I will only make suggestions of what could be the issue.
Because the Messaging Application Program Interface (MAPI) exception states "NotFound," this indicates that the source or target database is not responding as expected. This could be something as simple as the Exchange Mailbox Replication Service is not running or information stores are not mounted. It may be something less obvious, for example Exchange mailbox corruption or invalid permissions. Confirm the scope of the issue: Is it happening to mailboxes on the same source and target databases versus any moves?
Since you are moving mailboxes between two different editions of Exchange, you may also want to make sure to apply the current versions and updates to eliminate any possible compatibility issues.
There is a documented issue with Exchange 2010 that could generate a "MapiExceptionNotFound: Unable to copy to target" error. However, this only applies to your Exchange 2010 server, and this issue was resolved with Update Rollup 3 for Exchange Server 2010 Service Pack 2.
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