Search is broken for a user’s mailbox in both the Outlook desktop application and in Outlook Web Access.
The recommended solution is to move the mailbox in Exchange Online.
New-MoveRequest Tony@StarkEnterprises.com
You monitor the status of this request using this command.
Get-MoveRequestStatistics -Identity Tony@StarkEnterprises.com
This process will take time. My experience is that a 40GB mailbox can take up to 10 hours to complete. Using this command with larger mailboxes still results in approximately 4GB/hr for the “message transfer rate”.
Looking for a solution to “no search results in Outlook”, I found this article.