In the main migrating mailboxes from Exchange On-premises to Exchange Online works nicely, and syncs to 95% all silently in the background. Then you have your cut-over task to reach 100%. We’ve migrated thousands of users with minimal if any interruption.
The chances are when you are nearing the end of your mailbox migration, this is when the anomalies and the troublesome items appear.
The latest issue I have experienced is locating a mailbox which was only about 450MB, but the Exchange Management Console was reporting 1.2 million items. Sort of OK if these items are spread around the mailbox, but this was kicking in the 1,000,000 folder item limit when we tried to migrate.
OK – slightly odd, lets do a bit more investigation with a Get-MailboxFolderStatistics against the user. Exported to CSV, and added all the mail items up. Well that returned 11,000 items. Where are the other 1.1 million items.
These clearly were not mail, calendar, contact items, as they would have shown in the Get-MailboxFolderStatistics results.
The Culprit – The Deferred Action Folder.
I located the problem by using tools including MFCMAPI – which can be used to view system folders, as well as the mailbox folders. The Deferred Actions folder is displayed in the screenshot – with more than 800000 items (I’d already started clearing the items)
The Deferred Action folder stores the action which would be carried out on a received message when the Outlook client is running. If this is a shared mailbox, or a mailbox which no client actually logs onto very often, then any client side rule processing will result in an action being added to the Deferred Action folder for processing the next time an appropriate MAPI client connects to the mailbox. Outlook Web Access will not process the rule, nor carry out the tasks of the rule. If allowed to build, then depending on the mailbox usage, and amount of messages, then this can build relatively quickly and approach the folder limit and prevent a successful mailbox migration
The Solution – MFCMAPI
Unfortunately there is no first party tool to solve this, but the very useful MFCMAPI tool by Stephen Griffin can be used to clear the items. https://github.com/stephenegriffin/mfcmapi/releases/tag/20.0.20192.02. Once the folder items are cleared, or reduced below the 1,000,000 individual folder item limit, then the mailbox will migrate.