What Is an OST File?

What Does OST Stand For?

OST stands for Offline Storage Table. It is the file format Microsoft Outlook uses to maintain a local, synchronized copy of a server-based mailbox. When Outlook is connected to an Exchange server, Microsoft 365, or an IMAP account, it creates an OST file on the local machine so that you can continue working with your email even when the network connection is temporarily unavailable.

The OST file acts as a two-way mirror: changes you make offline (reading, composing, deleting) are synced back to the server when connectivity is restored, and new server-side messages are downloaded to the OST when you reconnect.

What Is Inside an OST File?

An OST file contains substantially the same data as the server mailbox it mirrors:

• Emails — all messages from the synchronized folders, including body content, HTML formatting, and attachments.
• Folders — the complete folder hierarchy as it exists on the server: Inbox, Sent Items, Drafts, Deleted Items, and custom folders.
• Contacts — the Contacts folder from the Exchange mailbox.
• Calendar events — all calendar items, meetings, and recurrence patterns.
• Tasks and notes — task lists and Outlook notes.
• Offline changes — any actions performed while disconnected (drafts, read status changes, deletions) are queued for synchronization.

How OST Differs From PST

The key distinction is portability. A PST file is a standalone archive that any Outlook installation can open. An OST file is cryptographically bound to the Outlook profile and Exchange account that created it. You cannot copy an OST file to another computer and open it in a different Outlook profile — Outlook will refuse. For a detailed comparison, see PST vs OST.

Where Are OST Files Stored?

Outlook stores OST files in a predictable location:

• Windows 10/11: C:\Users\<username>\AppData\Local\Microsoft\Outlook\
• The filename typically matches the email address or account name (e.g., user@company.com.ost or user@company.com - user.ost).

OST files can become very large. A mailbox with 10 GB of data on the server can produce an OST file of 10-15 GB locally, since the OST includes additional indexing structures.

When Do You Need to Access an OST File?

In normal operation, you never interact with the OST file directly — Outlook manages it transparently. However, several situations require direct access:

• Employee departure — the Exchange account is disabled, but the OST file on the laptop contains important emails.
• Exchange server failure — the server is down and you need to extract emails from the local cache.
• Forensic investigation — an OST file recovered from a hard drive needs to be examined as evidence.
• Profile corruption — the Outlook profile is damaged and you cannot reconnect to Exchange, but the OST file is intact.

How to Open or Convert an OST File

Since OST files cannot be opened directly by another Outlook profile, conversion is the standard approach.

1. Upload the OST file to mailtopst.com.
2. Choose your target format — PST (for Outlook), EML (for Thunderbird, Apple Mail), or MBOX (for archival tools).
3. Download the converted file and open it in the appropriate application.

MailtoPst reads the OST binary structure directly, regardless of whether the original Exchange account still exists. Files are encrypted via TLS during transfer, processed on EU servers, and automatically deleted afterward. Fully GDPR compliant.

Upload your file now and convert your OST — try free, no credit card required.

Frequently Asked Questions

Can I rename an OST file to PST and open it?

No. Despite sharing a similar binary architecture, OST and PST files have different internal flags and encryption settings. Renaming the extension does not change the file structure, and Outlook will reject it.

What happens to my OST file if I delete my Outlook profile?

The OST file remains on disk but becomes orphaned — no Outlook profile claims it. You can still convert it using MailtoPst, but Outlook itself will not open it.

Can I reduce the size of my OST file?

Outlook automatically manages OST size based on the Cached Exchange Mode settings (how many months of email to cache). You can reduce the cache period in Account Settings to shrink the OST. Compacting via Outlook (Account Settings, Data Files, Settings, Compact Now) can also reclaim unused space.

Is my OST file encrypted?

OST files use a form of encryption that is tied to the Outlook profile. This is not strong cryptographic protection — it is primarily designed to prevent casual access. MailtoPst can read this encryption as part of the conversion process.

What if my OST file is corrupted?

Minor corruption can sometimes be repaired by Outlook’s built-in recovery when reconnecting to the server. For orphaned or severely corrupted OST files, MailtoPst attempts to extract as much data as possible during conversion and reports any messages that could not be recovered.

Upload your file now and recover the contents of your OST archive.