I have one out of 150 mailboxes in a cutover migration, where the Recoverable Items folder has grown to over 2.5 million (4 GB) items - It was 1.5 million early last week. The migration is effectively waiting on this single mailbox to complete but it seems to be growing so quickly that it may never catch up - but I am not sure as we have migrated 1 million of this mailbox's items to the cloud.
The sheer amount of messages seems to indicate that they aren't valid messages or items that the user is deleting in Outlook. He has only worked with the company for 2 years - it is highly unlikely that the user had that many messages/items.
Source Exchange is: SBS 2011 SP1
Target: Exchange Online (Office 365)
Thank you in advance.