Recover Outlook Tenant Email History

During a recent split of two businesses that once shared the same Microsoft 365 tenancy, all historic emails for one of the Outlook accounts disappeared. The mailbox had always been used through the desktop version of Outlook and—unfortunately—no backups or archives were created beforehand. I need an expert who knows the ins and outs of Microsoft 365, Exchange Online, eDiscovery, retention policies, and related PowerShell commands to locate and restore every missing message (or at the very least export them to a usable PST). If you have proven experience recovering mailboxes after tenant migrations or accidental deletions, please outline your plan of attack and the estimated timeframe to get those messages back. The task is complete when I can open Outlook on the desktop client and see the full historical email trail restored, or receive a PST/OST that contains the recovered data ready for import.

Project ID: 40564161