Mailboxes contain a new folder named "Recoverable Items" and it's filling the users mailbox
Source Exchange 2010 Dumpster is being synchronized to the target as "Recoverable Items" in users Mailboxes. The Synchronization of the Exchange Dumpster is not supported, as noted in the Migration Manager 8.7 Release notes, detailed in the Additional Information section of this article.
When the data is synchronized to the target, the target does not know what to do with it, other than create the "Recoverable Items" folder.
This folder is not touched by the "Deleted Items Retention Time" either, therefore the users mailboxes are filling up, and exceeding storage limits.
There are no true solutions at this time.
The suggestion is to purge the Exchange 2010 Dumpster before performing any mailbox or Calendar Synchronization.
This can be accomplished safely by adjusting the retention time to 0 days. Or, this can be adjusted to purge after successful back. See the following article for further details.
This is due to be repaired in a future version.