Step-by-step repairing an Outlook PST file and then migrating the data to Office 365 involves two clear phases: fixing corruption with tools like ScanPST and then importing the healthy PST into Microsoft 365.
Understanding PST issues
A PST (Personal Storage Table) file stores Outlook emails, contacts, calendars, tasks, and other mailbox data locally. Over time, large file size, abrupt shutdowns, add-in conflicts, or disk errors can corrupt the PST, causing Outlook crashes, missing items, or send/receive failures.
Phase 1: Prepare and back up
Before any repair, close Outlook and create a copy of the PST file so you can roll back if something goes wrong. You can locate the active PST from Outlook via File > Account Settings > Data Files, then note the path and copy the file to a safe backup folder.
Phase 2: Repair PST with ScanPST
Microsoft’s Inbox Repair Tool (ScanPST.exe) is built into Office and is the first method to repair mild to moderate corruption. Once you find and launch ScanPST, browse to the PST file, start the scan, and then choose Repair when errors are detected, optionally keeping the default backup option enabled.
When ScanPST is not enough
ScanPST focuses on internal consistency and may fail with very large or heavily damaged PST files. In that case, many admins use third‑party repair utilities that can rebuild PST structures more aggressively and then generate a clean PST for import.
Validate Outlook after repair
After ScanPST (or any repair), reopen Outlook and attach the repaired PST if needed, then verify folders, search, and send/receive. If items appear in a special “Recovered” folder, drag or copy relevant messages, contacts, and calendar items back into your primary mailbox structure.
Phase 3: Choose migration approach
For migrating repaired PST data to Office 365, small environments can import directly from the Outlook client, while larger organizations should use Microsoft 365’s centralized PST import options. The two most common routes are manual Outlook Import/Export for individual users and the network upload method via the Microsoft Purview (compliance) portal for bulk migrations.
Option 1: Import via Outlook
On a user workstation, open Outlook with an Office 365 profile, then use File > Open & Export > Import/Export to import from an Outlook Data File. Select the repaired PST, include subfolders, choose the target Office 365 mailbox root, and select “Do not import duplicates” to keep the cloud mailbox clean.
Option 2: Network upload to Office 365
For many PSTs, Microsoft recommends uploading them to an Azure storage area using the AzCopy tool and then mapping them to mailboxes. You assign the Mailbox Import Export role, create an import job in the Purview portal, upload PSTs with AzCopy using the SAS URL, then provide a CSV mapping file and start the import.
Monitor and verify migration
After starting the import job, monitor its status in the Purview or Exchange admin center until it completes. Once finished, spot‑check user mailboxes in Outlook on the web and the desktop client to confirm folder hierarchy, message counts, and search behavior look as expected.
Best practices for stability
To avoid future PST problems, keep PST sizes under control, maintain reliable backups, and prefer native Office 365 mailboxes over long‑term PST storage. For ongoing migrations, standardize a repeatable process: repair locally, validate, upload or import, verify in Office 365, and document every batch.