FIX: Outlook .OST data file exceeds 50 GB limit with Shared Mailboxes.
If you are a Microsoft/Office 365 administrator and find out that the Outlook Offline Data File (.OST) of users accessing shared mailboxes exceeds the 50 GB limit, read this guide that contains effective ways to reduce the size of the Outlook .OST file.
As you may know, when you use the Cached Exchange mode in Outlook for user or shared mailboxes, all the folders and items of the mailboxes are downloaded and stored locally in the user's Outlook .OST data file.
However, although this helps Outlook run faster because it reads data from the local hard drive instead of connecting to the Microsoft Exchange Server to retrieve shared mailbox data, on the other hand, it dramatically increases the size of the .OST data file and causes Outlook to be unable to send or receive new email messages if the size of the .OST file size exceeds 50 GB. To resolve this issue, follow the instructions below.
How to reduce the size of the local .OST data file when working with Shared Mailboxes in Microsoft Outlook (Microsoft/Office 365).
Method 1. Reduce the amount of mail to keep locally (Offline).
The first method to reduce the size of your Outlook .OST file is to reduce the number of emails you want to be available even when you are offline. To do that:
1. Go to File > Account Settings > Account Settings.
2. Double click on your Exchange Account to open its settings.
3. Under Download email for the past, drag the slider to the left to download locally the emails of a smaller time period (e.g from the last 6 or 3 months). Then click Next and Done.
4. Finally, restart Outlook to apply the change. Upon restart, Outlook will create a new OST file in which it will download and store only the messages from the selected period.
Method 2. Disable Caching of Shared folders.
By default, Microsoft Outlook downloads shared mailboxes and folders for offline access, which can cause quite a few problems and can dramatically increase the size of the Offline Outlook Data (.OST) file. To prevent shared mailbox data from being downloaded locally on the .OST data file, proceed and disable the Shared Folder's caching:
1. Go to File > Account Settings > Account Settings.
2. Double click on your Exchange Account to open its settings.
3. Click More Settings
4. Go to the Advanced tab and uncheck the Download shared folders option. Then click Apply > OK. Then click Next and Done.
5. Finally, restart Outlook to apply the change.
Method 3. Disable Shared Folders Automapping in Microsoft Outlook.
By default, after users are granted access to a shared mailbox, that mailbox will be automatically added/mapped to their Microsoft Outlook client and will appear in Outlook's folders list after restarting Outlook.
However, because this can cause a significant increase in the size of your Outlook .OST data file, another approach to keeping your .OST file size small, is to disable the automatic mapping of the Shared Mailbox in Microsoft Outlook and access the shared mailbox data only Online, through the Outlook for Web (OWA) client.
To prevent the automapping of a shared mailbox in Microsoft Outlook, follow the instructions found at this tutorial: How to Disable the Automapping of a Shared Mailbox in MS Outlook in Exchange Online (Microsoft 365).
Method 4. Move Old Emails to Mailbox Archive.
If, after following the above methods, the size of the Shared Mailbox continues to grow significantly, then you can free up space by moving old mail to the Mailbox Archive either manually or automatically using a default retention policy or a custom retention policy that you have created in the Microsoft Purview portal.
Info: The Mailbox Archive (aka "Online Archive" or "In-Place Archive") in Microsoft/Office 365 provides users with additional storage space* to archive their older emails, helping to keep the size of their primary mailbox under control.
When you enable the Mailbox Archive on a Shared mailbox without a license, you'll get an additional 50 GB of storage space for archiving. If you want almost unlimited storage (1,5TB) for archiving, you need to assign to the mailbox a Microsoft 365 Business Premium, or a Microsoft 365 Enterprise E3/E5 license. (Source)
Step 1. Enable "Mailbox Archive" on the Shared Mailbox.
1. Navigate to Exchange Online Admin Center and go to Recipients > Mailboxes.
2. Click on the Shared Mailbox you want to enable the archiving.
3a. Select the Others tab and then click Manage mailbox archive.
3b. Turn On the Manage mailbox archive switch and click Save.
4. Now wait 1-2 hours for Mailbox Archive to be enabled, then restart Microsoft Outlook until you see a new folder in Outlook folders list named "Online Archive – %Shared Mailbox Name%", or a folder named "In-Place archive" in the Outlook for web. (OWA). When, this done continue to step-2.
Step 2. Move Old Email to Online Archive (In-Place Archive)
After you enable archiving in the Shared Mailbox, advise your users to move their old emails to the Mailbox Archive, either manually (by selecting the old items and moving them to mailbox archive), or automatically by assigning a default retention policy*.
* For example: To automatically move messages older than 1 year from your Inbox to a folder with the same name in your In-Place Archive (Online Archive), do the following in the new Outlook client or in Outlook for web (OWA):
1. Right-click on the Inbox folder and then select Assign policy > Personal 1 year move to archive.
Additional help.
If after following the above methods you still have problems with the size of the Outlook .OST data file, you can do one of the following:
- Bypass the 50 GB limit of the local Outlook .OST data file by increasing its limit via the registry. (The downside of this is that if the Outlook data file grows too large, for example, to 100 GB, causes performance issues in the Outlook application).
- Assign to the Shared Mailbox a Microsoft 365 Business Premium, or a Microsoft 365 Enterprise E3/E5 license (Source) to increase its storage limit to 100GB and then increase also the .OST size limit in client computers via registry.
- Create a local Outlook data file and move the old emails there. (Keep in mind that the disadvantage to this is that the data will be stored locally on a single computer and will not be available to all users who have access to the shared mailbox).
That's it! Which method worked for you?
Let me know if this guide has helped you by leaving your comment about your experience. Please like and share this guide to help others.
Frequently Asked Questions
What causes the Outlook .OST file to exceed its size limit?
Outlook .OST files can exceed the 50 GB limit when using Cached Exchange mode, as all folders and items from shared mailboxes are downloaded and stored locally, increasing the file size.
How can I reduce the size of the Outlook .OST file?
You can reduce the size of the OST file by decreasing the amount of mail kept offline, disabling caching for shared folders, disabling shared folder automapping, or moving old emails to a Mailbox Archive.
What is the benefit of reducing the amount of mail stored offline in Outlook?
By reducing the mail stored offline, Outlook's OST file size decreases, allowing it to send and receive new emails efficiently by only storing emails from a shorter, specified period.
How do I disable automatic mapping of shared mailboxes in Outlook?
Disabling automatic mapping of shared mailboxes can be done by following the instructions in a specific tutorial about disabling automapping for shared mailboxes in Exchange Online. This helps keep the .OST file smaller.
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