Uploading an OST file to Office 365 Exchange Online is a common requirement when users move from on-premises Exchange, lose access to an old mailbox, or face profile corruption. OST (Offline Storage Table) files store a synchronized copy of mailbox data for offline access in Outlook. While they are designed for temporary caching, OST files often become the only source of critical emai... https://blogosm.com/how-to-upload-an-ost-file-to-office-365-exchange-online