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Last updated: April 8, 2026

Quick Answer: Deleting an OST file is generally safe and can even be beneficial for troubleshooting Outlook performance issues. However, it's crucial to ensure that Outlook is closed and that you have a proper backup of your data if you wish to retain it. The OST file is a local cache of your mailbox data, and Outlook will automatically recreate it when the application is reopened.

Key Facts

An OST file is an Offline Storage Table, a local cache for Microsoft Outlook data.
Deleting an OST file will force Outlook to create a new one upon next launch, resynchronizing with the mail server.
This process is safe and often resolves issues related to Outlook performance, corruption, or synchronization problems.
Always ensure Outlook is closed before attempting to delete an OST file to prevent data corruption.
If you need to preserve specific data that might not be fully synchronized or if you're dealing with IMAP accounts where the OST is the primary source of data, consider exporting your mailbox to a PST file first.

Overview

The .ost file, or Offline Storage Table, is a crucial component for Microsoft Outlook users who utilize Exchange, Microsoft 365, Outlook.com, or IMAP accounts. This file acts as a local cache, mirroring the contents of your mailbox on the mail server. This mirroring allows you to access your emails, calendar, contacts, and tasks even when you're offline, and it significantly speeds up operations by reducing the need to constantly communicate with the server. For POP accounts, a .pst file (Personal Storage Table) is typically used, which is a separate entity and not a synchronized cache.

Many users ponder the safety of deleting this OST file. The short answer is often yes, it is safe, and in many cases, it's a recommended troubleshooting step. When Outlook is closed, deleting the OST file essentially tells the application to discard its current local copy of the mailbox data. Upon the next launch, Outlook will establish a new connection with the mail server and generate a fresh OST file, redownloading and resynchronizing all your mailbox items. This clean slate can resolve a myriad of issues, from slow performance and unresponsiveness to synchronization errors and profile corruption.

How It Works

Understanding the OST File: The OST file is not an archive of your emails; it's a synchronized copy. When you make changes in Outlook (e.g., sending an email, moving an item to a folder), these changes are first written to the OST file and then synchronized with the mail server. Conversely, new emails arriving on the server are downloaded to the OST file. This ensures that your local view of the mailbox is as up-to-date as possible.
The Deletion Process: Deleting an OST file is akin to clearing Outlook's local memory. Imagine your computer has a temporary workspace; deleting the OST is like clearing that workspace. When Outlook restarts, it needs this workspace to function, so it automatically creates a new, empty OST file. It then communicates with your mail server to repopulate this new file with all your mailbox content.
Resynchronization: This is the critical step following deletion. Outlook connects to your mail server (Exchange, Microsoft 365, IMAP) and begins downloading all your emails, calendar entries, contacts, and other mailbox items into the newly created OST file. The time this takes depends on the size of your mailbox and your internet connection speed.
Benefits of Deletion: Corrupted OST files are a common cause of Outlook problems. Over time, data can become inconsistent or damaged, leading to errors. Deleting the OST and forcing a resynchronization can effectively resolve these issues, restoring Outlook to a stable working condition without losing any actual mailbox data from the server.

Key Comparisons

Feature	OST File	PST File
Purpose	Offline cache for Exchange, M365, Outlook.com, IMAP accounts	Archive or local storage for POP accounts or exported data
Synchronization	Continuously synchronizes with the mail server	Generally static unless used for manual exports/imports
Data Loss Risk on Deletion (with active account)	Low; data is on the server and will be redownloaded	High if it contains current mailbox data and is not backed up
Recreation	Automatically recreated by Outlook upon restart	Not automatically recreated; must be manually created or imported

Why It Matters

Troubleshooting Common Outlook Issues: Many users encounter problems like Outlook freezing, crashing, or failing to send/receive emails. Before resorting to more complex solutions, deleting the OST file and letting Outlook rebuild it is a highly effective first step. It's a simple yet powerful way to fix issues stemming from a corrupted local cache.
Ensuring Data Integrity: While the OST file is a local copy, your primary data resides on the mail server for Exchange, M365, Outlook.com, and IMAP accounts. Deleting the OST doesn't delete your emails from the server. Therefore, this action is safe in the sense that your mail history and critical data are preserved on the provider's end. For POP accounts, however, data is stored locally in a PST file, and deleting that without a backup would result in data loss.
Performance Enhancement: A bloated or corrupted OST file can significantly slow down Outlook. By deleting it and allowing Outlook to create a fresh, smaller file initially, you can often see a noticeable improvement in performance. This is especially true if the OST file has grown excessively large over time.

In conclusion, deleting an OST file is a safe and often beneficial procedure for users of synchronized Outlook accounts. It serves as a robust method for resolving a wide array of common Outlook glitches and performance degradations. The key is to ensure Outlook is closed beforehand and to understand that your mailbox data is safely stored on your mail server, ready to be downloaded anew. For those using POP accounts or wishing to archive data, the concept of PST files and their handling differs significantly, and a cautious approach with proper backups is always advised.