Why OST File Is Not Syncing on Mac & How to Fix It Quickly
Neeru Kuchhal
April 23rd, 2026
User-Query:
“My Outlook mailbox on Mac is not updating, and I’m not receiving new emails even though the internet is working fine. It looks like the OST file not syncing Mac properly. How can I fix this issue quickly without losing my mailbox data?”
— Posted by “Mia Anderson”
Summary: Is your OST file not updating on Mac? It can be frustrating when Outlook stops updating emails, calendar entries, or contacts properly. You may notice missing messages, delayed inbox updates, or folders not refreshing at all. However, many Mac users face this issue when using Outlook with Exchange, Office 365, or IMAP accounts.
The good news is that OST sync issues are mostly fixable. In most cases, the problem is related to internet connectivity, account settings, Outlook cache problems, or outdated software. To fix OST not syncing error, we are here with some common reasons and how to fix it quickly.
An Overview of the OST File
An OST (Offline Storage Table) is created by Microsoft Outlook when you connect your account with an email platform. The file stores a local copy of your mailbox data, such as emails, contacts, calendars, notes, and tasks.
This allows you to access your mailbox even without an internet connection. Once the internet is available again, Outlook automatically syncs the changes with the mail server. However, when the OST file not syncing Mac, Outlook may show outdated emails or fail to receive/send new data.
OST File Sync Stopped Working on Mac: Here are the common Reasons
- Weak or unstable internet connection.
- Outlook account authentication problems.
- Corrupt Outlook cache or profile data.
- Outdated Outlook for Mac version.
- Large OST file causing slow sync.
- Exchange or Office 365 server issues.
- Incorrect account settings.
- MacOS compatibility problems.
- Background sync is paused in Outlook.
- Damaged Outlook identity or database.
Is Your Mac Outlook Stuck? Try These Quick Sync Repairs
Now, let’s go through some quick methods that help users to fix OST sync problem on Mac.
Method 1: Check Your Internet Connection
A poor connection is one of the main reasons sync stops working.
- Open Safari or any browser and test a website.
- If pages are loading slowly, restart your Wi-Fi router.
- Reconnect to the network.
- You can also test using another Wi-Fi or hotspot.
Once the connection becomes stable, Outlook may begin syncing automatically.
Method 2: Restart Outlook for Mac
Sometimes Outlook simply gets stuck in the background.
- Close Outlook completely.
- Wait a few seconds.
- Open Outlook again.
This refreshes the app and may restart the sync process.
Method 3: Verify Account Credentials
If your password was changed recently, Outlook may fail to sync.
- Open Outlook on Mac.
- Go to Tools and then Accounts.
- Select your email account.
- Re-enter the password if needed.
Incorrect credentials often stop OST updates.
Method 4: Update Outlook and macOS
Older versions may create compatibility or sync problems.
- Open App Store or Microsoft AutoUpdate.
- Install the latest Outlook updates.
- Also, check for available macOS updates.
Keeping both updated helps Outlook run smoothly.
Method 5: Rebuild Outlook Profile
A damaged profile can block synchronization.
- Close Outlook.
- Open Outlook Profile Manager.
- Create a new profile.
- Add your account again.
This often fixes corrupted local sync data.
Method 6: Check Server Status
Sometimes the issue is not on your Mac.
If you are using Microsoft 365 or Exchange, the server may be temporarily down. Wait a little while and try syncing again later.
Conclusion
In this guide, we discussed why an OST file not syncing Mac and how to fix it quickly. Most of the time, the issue is caused by connection problems, outdated software, or account errors. Basic fixes like restarting Outlook, checking credentials, or updating the app usually solve the problem. However, if the issue continues or your Outlook data becomes inaccessible, keeping a backup is always the safest option.
Important Note: You may also convert OST to PDF seamlessly if needed.
Frequently Asked Questions
Ans. Sometimes, after a macOS or Outlook update, old settings may conflict with the new version. This can interrupt mailbox synchronization or slow down Outlook performance. Updating Outlook again or re-adding the account usually helps fix the issue.
Ans. Yes, if the local Outlook data file becomes damaged, Outlook may fail to sync emails, contacts, or calendars properly. You might notice missing messages or folders not updating. Rebuilding the profile or creating a new account profile can often resolve it.
Ans. Yes, you can usually access previously downloaded emails stored in the offline cache. However, new emails or recent changes may not appear until syncing starts again. The OST file mainly lets you view old mailbox data offline.
Ans. Reinstalling Outlook can help if the app files are damaged or outdated. It often resolves software-related sync issues, but it may not fix server or account setting problems. Before reinstalling, it’s best to check settings and back up important mailbox data first.
About The Author:
I am Neeru Kuchhal, a Technical Content Writer who focuses on turning complex ideas into clear, engaging, and impactful content. I combine creativity with strategy to write blogs, web content, and technical articles that connect with the right audience and support brand goals. I believe great content is not just written. It is thoughtfully created to inform, build trust, and drive action.
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