Exchange Online – How to Completely Re-Create User Mailbox

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

When you run the Get-MailboxPermission in Exchange Online, it can list all users who have mailbox level permission to the specific mailbox. For example:

For the IsInherited=True results, we can check the User column. Some permissions are interited from some role groups in AD and Exchange organization such as Administrator, Domain Admins, Organization Management, Enterprise Admins and so on.

These permissions are generally configured be default. In Exchange Admin Center, you can click Permissions > Admin Roles to view the role group.