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Last updated: April 8, 2026

Quick Answer: Deleting an OST file (Offline Storage Table) in Microsoft Outlook is generally safe if Outlook is configured to use an Exchange server or Microsoft 365 account and the OST file is not corrupted. This file is a local cache of your mailbox data and can be safely recreated by Outlook if needed. However, it's crucial to ensure you have a backup or are certain you don't need the offline data before proceeding.

Key Facts

OST files are local caches for Exchange, Microsoft 365, and IMAP accounts, storing mailbox data for offline access.
Outlook can automatically recreate an OST file if it's deleted, provided the account is still configured.
Deleting an OST file is safe for Exchange and Microsoft 365 accounts as the primary data resides on the server.
For POP accounts, the primary data is in an PST file; deleting the OST (if one exists) might not have adverse effects, but PST is the critical file.
Corrupted OST files can sometimes cause Outlook issues, and deleting them can be a troubleshooting step.

Overview

Microsoft Outlook uses various file formats to store your email, calendar, contacts, and other mailbox data. Among these, the Offline Storage Table (OST) file plays a significant role, particularly for users connected to Microsoft Exchange servers, Microsoft 365, or IMAP accounts. The primary purpose of an OST file is to provide users with offline access to their mailbox data. This means that even when you're not connected to the internet or the mail server, you can still view, compose, and manage your emails. Outlook achieves this by creating a synchronized copy of your mailbox on your local computer.

The question of whether it's safe to delete an OST file is a common one for Outlook users. The answer, however, is nuanced and depends heavily on the type of email account you are using and the current state of the file. Understanding the role of an OST file within Outlook's architecture is crucial before making any decisions about its deletion. Incorrectly deleting a vital data file can lead to data loss, while strategically removing a corrupted or redundant OST file can resolve performance issues and free up valuable disk space.

How It Works

Synchronization with Mail Servers: An OST file acts as a synchronized replica of your mailbox data stored on an Exchange, Microsoft 365, or IMAP server. When Outlook is running and connected to the internet, it continuously synchronizes changes between the OST file and the server. This ensures that your local copy is always up-to-date with the server-side mailbox. For Exchange and Microsoft 365 accounts, this synchronization is bidirectional.
Offline Access Capabilities: The primary benefit of an OST file is enabling offline access. Even without an internet connection, you can access all your emails, calendar entries, contacts, and tasks that have been downloaded to the OST file. Any emails you compose while offline will be queued and sent automatically once Outlook reconnects to the server.
Creation and Re-creation: When you set up an Exchange, Microsoft 365, or IMAP account in Outlook, the application automatically creates an OST file for that account. If an OST file is deleted or becomes corrupted, Outlook can typically recreate a new one the next time you launch the application and connect to the mail server. This re-creation process involves re-downloading your mailbox data from the server.
Account Type Dependency: It's critical to distinguish OST files from PST (Personal Storage Table) files. PST files are used for POP accounts and also as archives for other account types. The primary mailbox data for POP accounts resides in the PST file. Deleting an OST file generally does not affect the data for a POP account, but deleting a PST file would result in data loss if it's not backed up.

Key Comparisons

Feature	OST File	PST File
Primary Use	Offline cache for Exchange, Microsoft 365, IMAP accounts	Primary storage for POP accounts; archive for any account
Synchronization	Synchronizes with server; data primarily resides on server	Does not synchronize with server; local storage
Re-creation after Deletion	Outlook can recreate if account is configured	Cannot be recreated if deleted; requires backup
Offline Access	Enables offline access to server data	Stores data locally; accessible offline
Server Dependency	Relies on server for master copy of data	Independent of server for data storage

Why It Matters

Data Integrity: For Exchange and Microsoft 365 accounts, the primary source of your mailbox data is the server. Deleting a correctly functioning OST file for these account types is generally safe because Outlook will simply create a new one and re-download your data. This means you won't lose any emails or important information, as it all exists on the server. The impact is primarily the time it takes to re-synchronize.
Troubleshooting Outlook Issues: Sometimes, OST files can become corrupted, leading to Outlook performance problems, errors, or crashes. In such scenarios, deleting the corrupted OST file and allowing Outlook to create a new, clean one can be an effective troubleshooting step. This process effectively resets the local cache, often resolving the underlying issues without any loss of server-side data.
Disk Space Management: OST files can grow quite large, especially if you have a substantial mailbox. If you're running low on disk space, and you've confirmed that your email account is set up correctly with a healthy server-side mailbox, deleting an OST file (and letting Outlook recreate it) can help reclaim significant storage. However, this should be a considered action, not a casual one, especially if you have slow internet or limited data allowances for re-downloading.

In conclusion, deleting an OST file is generally safe for users with Exchange, Microsoft 365, or IMAP accounts, provided the file is not corrupted and you are comfortable with Outlook re-downloading your mailbox data. The key is to understand that the OST file is a cache, not the primary storage for these account types. However, always exercise caution, ensure you have a reliable internet connection for re-synchronization, and consider backing up critical data if you have any doubts. For POP accounts, focus on your PST files, as they contain your primary data.