PST Files and Your Migration: Find Them, Migrate Them, Retire Them

Every email migration project eventually collides with PST files. They are the archive format Outlook has used for decades, and in most companies they are everywhere: on laptops, on file shares, on USB drives in desk drawers, and attached to Outlook profiles that nobody has reviewed in years. A migration that moves the server mailboxes and ignores the PSTs has moved the visible mail and left the shadow archive behind.

This guide covers why PSTs accumulate, why they are a genuine risk rather than a harmless quirk, how to find them, the three realistic ways to get them into Microsoft 365, and how to make sure new ones stop appearing.

Why you have PSTs, and why they are a problem

PSTs exist mostly because of old mailbox quotas. When on-premises Exchange servers enforced small mailboxes, users were told (often by IT) to "archive to a PST" to free space. AutoArchive did it automatically for many. Departing employees' mail was exported to PSTs "just in case." Twenty years later, the result is an uncatalogued, unmanaged archive of company email scattered across endpoints.

The problems compound:

• Corruption. PSTs are a single-file database format that does not tolerate interruption well. Files on network shares, files that grew huge under old format limits, and files on aging disks all corrupt, and users typically discover this the day they need something from 2016.
• No backup. A PST on a laptop's local drive is usually outside every backup system you have. When the laptop dies, that mail is gone.
• Discovery and compliance risk. If your business faces a legal hold or a records request, email in PSTs is still discoverable, but you cannot search it centrally, cannot place holds on it, and may not even know it exists. "We could not find it" is not a defensible position when the file was sitting on a share.
• Security exposure. PSTs walk out the door on USB sticks and in personal cloud folders. They are a data-loss channel with no logging.

A migration to Microsoft 365 is the natural moment to fix all of this, because you are already touching every mailbox and every device.

Step 1: Find them

PST discovery has two layers. The first is technical: scan endpoints and file shares for *.pst files, and inventory which PSTs are attached to live Outlook profiles. Attached PSTs matter most, because users actively rely on them and will notice immediately if they vanish. The second layer is human: ask. Department heads know about the "shared" PST on the finance drive that three people open in turn. Long-tenured assistants know where the old executive archives live. A short survey catches files no scan reaches, like the ones on home machines.

The output of discovery is a register: file path, owner, size, last modified date, and whether it is attached to a profile. That register drives every decision that follows, including the honest one: some PSTs are duplicates of each other or of mail that already exists in the mailbox, and some are old enough to fall outside your retention obligations and can be defensibly deleted rather than migrated. Migrating garbage costs storage and attention forever.

Step 2: Choose an import route

There are three realistic ways to get PST content into Microsoft 365, and the right one depends on volume:

For network upload and drive shipping, decide deliberately whether each PST lands in the user's primary mailbox or their online archive. Archives are usually the right destination for old material: it keeps primary mailboxes lean and search fast, and archive capacity is generous on the plans that include it. Our guide to mailbox and item limits in Microsoft 365 covers the quota math, including what happens to oversized items, which simply skip during import.

Step 3: Handle the ugly ones

Expect a percentage of discovered PSTs to be damaged. Options in rough order of preference: repair with the Inbox Repair tool (scanpst) for light corruption, copy the readable content out via Outlook for moderate damage, and commercial recovery tooling for the files that matter enough to justify it. Decide in advance, with the business, which files are worth recovery effort and which get documented as unrecoverable. Doing this during the migration, with a project structure around it, beats doing it during a lawsuit.

Also expect password-protected PSTs (the protection is weak, but the password still has to be dealt with), PSTs in the ancient ANSI format with its 2 GB ceiling, and files whose owners left the company years ago. Each needs a documented decision, not an improvised one.

Step 4: Retire the format

Importing the backlog fixes the past. Retiring the format fixes the future, and it is the step most migrations skip:

1. Remove the reason PSTs existed. Microsoft 365 mailboxes are large, and online archives with retention policies replace AutoArchive entirely. Users no longer need local archives, but only if you turn the server-side equivalents on.
2. Block new PST growth. Group Policy and Intune settings can prevent Outlook from adding new PST files or writing new mail into existing ones. Roll this out after the import, not before, so users are never stranded.
3. Delete the source files. After verified import, the original PSTs should be removed from endpoints and shares, with the register updated to show what was imported, what was deleted, and what was intentionally retained. Leaving them in place quietly recreates the discovery problem you just paid to solve.
4. Tell people what changed. A short note explaining where archived mail now lives (the online archive folder in Outlook) and how to search it prevents most of the follow-up tickets.

Where a specialist changes the outcome

The individual steps here are not exotic. What a migration partner brings is the discipline across hundreds of files and dozens of users: a discovery process that actually finds the shadow archives, a mapping file that puts each PST in the right mailbox and folder the first time, a defensible record of what was imported, repaired, or deleted, and the policy work that stops the PST population from regrowing. We fold PST remediation into the same fixed-scope plan as the mailbox move itself, whether you are coming from an IMAP or legacy system, on-premises Exchange, or Google Workspace.