Originally found this post on the Technet forums which links to this MS Exchange Team blog from a few years ago.

This cmdlet is great for discovering or removing messages by searching one or many mailboxes. Excellent for removing large messages sent to large distribution groups or virus/spam messages if identified early by the administrator, etc.

A couple of errors that might be returned by this cmdlet are:

[0] [ERROR] Error was found for (SourceMailboxEmailAddress) because: Error occurred in the step: Moving messages. Failed to copy messages to the destination mailbox store with error:
MAPI or an unspecified service provider.
ID no: 00000000-0000-00000000, error code: -1056749164

[0] [ERROR] Error was found for (SourceMailboxEmailAddress) because: Error occurred in the step: Creating target folder in the target mailbox. An unknown error has occurred., error code: -2147221233


These are permissions issues. The account running the command needs to have full permissions to the source mailbox (first error code) and the destination mailbox (second error code). This is explained well here

That said, also take a look at this if you are sure that permissions are set correctly