The first thing is validate your public DNS on Microsoft’s tenant. Insert you DNS domain The tenant tells us which records we’ll need to add to our public DNS Do it . When you have done these changes on your public DNS, We export the pst from the old system (See this guide: https://marcoschiavon.net/2018/06/07/how-to-export-a-pst-from-an-exchange-2010-with-powershell/ ) …