Hi @mdwsgl ,
Considering that this problem occurs with only one linked mailbox, have you tried to export the linked mailbox to a .pst file, create a new user mailbox in Exchange 2010, and import the .pst file to this new user mailbox. Then migrate this user mailbox to Exchange online. For a single mailbox, I think this is the most effective way.
For more information you could refer to: Create a Mailbox Export Request and Create a Mailbox Import Request
In addition, due to the particularity of linked mailboxes, if you want to know more about the settings of linked mailboxes and Exchange online, you can refer to: EXCHANGE RESOURCE FOREST AND OFFICE 365 – PART I
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
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