How Outlook Stores Mailbox Data in OST Files

Cached Exchange Mode and the role of the OST file

When Cached Exchange Mode is enabled, classic Outlook keeps supported mailbox data available locally in an OST and periodically synchronizes changes with the live Exchange or Microsoft 365 mailbox. Users can read cached items and create offline changes, but server-dependent features and content that has not been cached still require connectivity.

The OST file is associated with the account and Exchange profile that created it. It cannot be opened independently by another Outlook profile — Outlook validates the file against the original mailbox and profile during each connection attempt, so it isn't portable the way a PST file is. A PST file, by contrast, is a portable, standalone container that Outlook can open without any server connection at all, which is one of the key structural differences between the two formats.

In Online Mode, Outlook relies more directly on the live server and does not maintain the full Cached Exchange Mode offline mailbox copy described on this page.

This explanation covers classic Outlook for Windows. New Outlook for Windows uses a different offline-storage model and doesn't follow the same Cached Exchange Mode/OST architecture described here.

Internal structure of an OST file

An OST uses a proprietary Outlook data-file format for locally cached mailbox folders and supported item properties. It is not an Exchange EDB database, and the two formats are not interchangeable. This page keeps the internal description at a user-facing level because implementation details should not be inferred from Exchange database terminology.

For format history and currently supported behavior, see the corrected ANSI and Unicode OST file formats guide and Microsoft's current Outlook data-file documentation.

Where OST files are stored on disk

By default, Outlook creates the OST file under the user's local profile, typically at a path resembling:

C:\Users\username\AppData\Local\Microsoft\Outlook\

The exact location and file name can vary by Outlook configuration and policy. Use Microsoft's current guidance or Outlook account settings to locate the active data file instead of treating one path or naming pattern as universal.

When the server mailbox is healthy and complete, Outlook can normally recreate the cache. Preserve the existing OST before deletion or profile removal if it may contain offline or local-only changes that have not synchronized.

How synchronization keeps the OST file current

Outlook synchronizes cached mailbox content and supported changes between the OST and the server. New server items can be downloaded to the cache, while offline changes can be uploaded when connectivity returns.

Offline and server-side edits can conflict when synchronization resumes. Outlook may record synchronization evidence in Sync Issues or related folders; the server or OST should not be assumed to be the source of truth in every conflict.

Cache-range and shared-folder settings affect what is stored locally. Classic Send/Receive settings do not act as one universal master timer for every Exchange Cached Mode update.

Folder-level storage and item retention

The OST can contain cached mail folders, messages, calendar items, contacts, tasks, and synchronization information. Online Archive content remains server-backed and should not be assumed to be part of the primary local OST cache.

When Cached Exchange Mode is combined with the "Download shared folders" or slider-based "Mail to keep offline" setting introduced in later Outlook versions, only items within the selected time window are stored locally, while older items remain retrievable from the server on demand. This partial caching behavior means the OST file does not always contain a complete copy of the mailbox, which is a relevant distinction when planning any local recovery scenario.

What an OST File Typically Contains

Within the limits described above, an OST file generally holds:

• Cached email folders and messages within the configured cache range.
• Calendar, contacts, tasks, and other supported mailbox data synchronized locally.
• Locally queued or offline changes awaiting synchronization with the server.

What May Not Be in the OST File

Because caching is selective and time-bounded, an OST file may not include:

• Older mail outside the configured local cache range.
• Online Archive mailbox content, which is stored and accessed separately from the primary OST.
• Server-side changes made after the last successful synchronization.
• Items that were never downloaded to the device in the first place.
• Some shared or delegated content, depending on the shared-folder caching settings in use (see below).

Shared Folders and Cached Exchange Mode

Cached Exchange Mode has separate controls for shared and delegated folders. The presence of shared content in an OST depends on those settings and synchronization state, not only on whether the primary mailbox uses Cached Exchange Mode.

OST vs PST at a Glance

Why an OST is not a mailbox backup

The normal data flow is: Exchange or Microsoft 365 mailbox ↔ Cached Exchange Mode synchronization ↔ local OST. The cache can be recreated while the live mailbox remains healthy, but it may omit older mail outside the configured range, Online Archive content, items never downloaded, and later server changes. Server retention and a managed backup strategy remain the correct protection layer.

An OST becomes valuable as a recovery source only when the live mailbox, profile, server, or better backup source is unavailable and the local cache may contain unique data.

Why OST files become inaccessible

An OST can become inaccessible when its original account or profile is removed, the mailbox is deleted, the cache is corrupted, storage errors damage the file, or synchronization is interrupted. These conditions do not prove that every item is lost or recoverable; the result depends on the live mailbox and what remains readable in the local file.

For example, after a mailbox is deprovisioned, an old workstation may retain the last locally cached OST. Outlook cannot attach that old cache to a different mailbox like a PST. If the file contains unique required data, preserve it and evaluate it as a recovery source.

Recreating an OST file versus recovering its contents

In most day-to-day scenarios where an OST file is flagged as corrupted while the source mailbox is still active, the standard remediation is to delete the local OST file and let Outlook rebuild it from scratch on the next connection. This works because the server remains the authoritative data source, and Outlook simply re-downloads the mailbox content into a fresh file. Microsoft's Inbox Repair Tool (SCANPST.EXE) can also attempt to fix structural inconsistencies in a damaged OST or PST file without discarding it, though its repair capability is limited to certain classes of corruption and does not restore items already flagged as unreadable.

Recreation is not a viable option, however, when the mailbox itself is no longer available on the server, when the goal is to recover the last known state before a mailbox was altered or deleted, or when the local file needs to be preserved as evidence or as an offline archive independent of any live Exchange connection. In these cases, the OST file has to be treated as a standalone data source, and its contents parsed directly, since there is no server-side copy left to re-synchronise from.

Accessing Stored OST Data Outside of Outlook

An OST is an account/profile-associated cache rather than a portable Outlook data file. Outlook does not provide a supported way to attach an arbitrary old OST to another profile or mailbox as it does with PST.

When the original mailbox cannot supply required data, a recovery application can scan the old OST independently and export only data that remains readable. Renaming or copying the file does not turn it into a PST.

EdbMails OST to PST Converter can scan and preview readable OST content and export selected data to PST when the live mailbox or original profile is unavailable.

 See the full step-by-step guide on how to convert OST to PST