What Is a PST File? Outlook's Local Mailbox Explained

A PST file is Outlook's local mailbox container. The acronym stands for Personal Storage Table. Unlike an OST, which is a cache, a PST is authoritative — if you delete the file, the data inside is gone. For two decades PSTs have been the default way people archive mail they wanted to keep but did not want cluttering the server, which is why almost every Exchange and Microsoft 365 migration eventually hits a pile of them. This entry covers what PSTs are, the difference between the two formats, and how to get the data inside them onto a modern platform.

The short definition#

A PST is a single binary file with the .pst extension that holds an entire Outlook data store: folders, messages, attachments, calendar entries, contacts, tasks, notes, and rules. Outlook can mount any PST as a separate folder tree in the navigation pane. You can attach more than one PST to a profile at a time.

Where an OST is tied to a specific Outlook profile, a PST is portable. Copy it to another machine, open Outlook, attach the file, and the contents appear as a new tree. That portability is the reason PSTs are everywhere — and also the reason they cause so many problems.

ANSI vs Unicode PST#

There are two PST formats in the wild.

• ANSI PST — the old format. Hard-capped at 2 GB. Outlook 2002 and earlier created these by default. Most ANSI PSTs in 2026 are old archives that nobody has touched in fifteen years.
• Unicode PST — the modern format. Default cap of 50 GB, raisable to roughly 100 GB via a registry value. Outlook 2003 and every version since creates Unicode PSTs by default.

The two formats are not interchangeable. ANSI PSTs cannot grow past 2 GB and will silently start refusing writes when they hit it. The standard fix is to create a new Unicode PST and copy the data across folder by folder.

Why PSTs are an admin headache#

A short list of the problems every Exchange admin can recite from memory:

• They sit on individual laptops and almost never on a backed-up share.
• Retention policies do not apply to them, so messages a compliance officer believes are deleted are still present in someone's archive.
• Legal hold and eDiscovery do not see inside them by default.
• They corrupt under network filesystems — Microsoft has explicitly told people not to store PSTs on a network share for two decades and people have ignored it for two decades.
• They lock to whichever Outlook process opens them, which causes "the file is in use" errors when two profiles try to attach the same PST.

Importing PSTs into Exchange or Microsoft 365#

For a small number of PSTs (under twenty, or under a few gigabytes), the Outlook desktop client handles it: File → Open & Export → Import/Export → Import from another program or file → Outlook Data File (.pst). Outlook walks every folder and uploads each item to the destination mailbox. Slow, but reliable, and it preserves folder structure.

For larger collections you use the Microsoft 365 Compliance portal import service. The two supported paths are:

1. Network upload — you AzCopy the PSTs to a temporary Azure storage container Microsoft provides, drop a mapping CSV that says which PST goes into which mailbox and folder, and the service ingests them server-side.
2. Drive shipping — for very large collections, you encrypt the PSTs onto a physical disk and mail the disk to Microsoft.

Both approaches are covered in detail in the PST to Microsoft 365 migration guide. For Gmail destinations, see PST to Gmail. For straight Outlook-to-Outlook moves see PST to Outlook.

PSTs in a broader migration#

When you are also dealing with MBOX and EML files, the strategy in the PST, MBOX, EML migration guide applies — normalize everything into the destination platform, preserve folder structure, then run a verification pass on item counts.

Two final tips. First, run scanpst.exe against every PST before you import it. Corruption rates in field-collected PSTs are higher than people expect, and a corrupt PST mid-import is harder to recover than a corrupt one you caught on a desk. Second, keep the original PSTs read-only and copy them before any import tool touches them. Some tools modify the source file on connection.