Powershell – How to extract specific emails from Exchange Online using PowerShell and move or sort them

Get-PublicFolderClientPermission and Get-PublicFolderAdministrativePermission will give you the permissions; Set-PublicFolder*Permission will let you apply them to the new hierarchy. Persisting the permissions is left as an exercise for the reader :)

Depending on how you are doing your first migration, it might be better to just import/merge everything during that step, rather than importing to separate mailboxes and then merging them after the fact.

To answer the question: You could use several methods to copy data from each mailbox into the 'primary' one for the user:

1. Using Search-Mailbox: Have Search-Mailbox go through the secondary mailboxes and specify the primary one as the target to copy the results to:

https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/search-mailbox?view=exchange-ps

2. Export the secondary mailboxes to a PST file, and then import them into the primary mailboxes:

Use the Compliance/eDiscovery search to export the data into a PST file: https://docs.microsoft.com/en-us/exchange/policy-and-compliance/ediscovery/export-results-to-pst?view=exchserver-2019

(N.B. If you're already exporting your data from your original server to PST files, you can use those.)

Then use the network upload / PST import part of the data migration facility to import that PST into the primary mailbox: https://docs.microsoft.com/en-us/office365/securitycompliance/use-network-upload-to-import-pst-files