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... (Budget: £250 - £750 GBP, Jobs: Active Directory, Data Management, Microsoft 365, Microsoft Exchange, Microsoft Outlook, Technical Support, Troubleshooting, Windows Server)