ost files: Increase size limit
This tutorial completes the caching “ Manage Outlook Cache Mode and OST File Size ”
I have a very large mailbox, and luckily, my Exchange Online mailbox is limited to 100 GB, which gives me more than enough storage space for my mailbox.
However, Outlook’s ost file is apparently much more limited as I’m getting sync errors and my ost file is “only” about 50GB.
I find it strange that they support 100GB mailboxes on Exchange when the maximum recommended ost file size for Outlook is still 50GB.
How can I increase the ost file limit and make Outlook support larger mailboxes?
In Outlook 2007, the maximum size of a Unicode ost file is 20 GB. In Outlook 2010, 2013, 2016, 2019, and Microsoft 365, this limit is 50 GB.
While you can increase this limit beyond 50GB, it is not advisable to do so due to possible performance issues. However, you are of course free to try and see if your computer can handle larger OST files.
You can increase the maximum ost file size through registry or group policy.
When your computer can’t handle larger OST files, you can still use other techniques to reduce OST file size. These techniques are discussed after the Registry and Group Policy method.
Note: File size settings and restrictions for OST files are managed using the same methods as for PST files. So, whenever you see “PST” in this guide, it also applies to OST files.
Method 1: Via the registry
To increase the maximum size of an ost file in Outlook, you need to create and set 2 values in the following location in the Registry;
- Outlook 2007
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST - Outlook 2010
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\PST - Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST - Outlook 2016 / Outlook 2019 / Microsoft 365
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
We need to create and define the following 2 new DWORD values;
- WarnLargeFileSize
Do not set it higher than 4090445042 (decimal) or f3cf3cf2 (hexadecimal). - MaxLargeFileSize
Do not exceed 4294967295 (decimal) or ffffffff (hexadecimal).
The first value is the number of MB a user can write to a PST file before receiving a warning that the file is full. The second value is the number of MB the system can write to a PST file. This difference should be at least 5%, because the data written to a PST file is not limited to the user’s data.
Method 2: Via Group Policies
The above registry settings can also be configured through Group Policy. To learn how to use Group Policy, see this guide.
The settings to adjust the size of the PST file are located in the Miscellaneous->PST Settings section. The following 2 settings need to be changed;
- Large PST: Absolute maximum size
Do not set it higher than 4294967295 - Large PST: Size to disable adding new content
Do not set it higher than 4090445042
Alternative: Techniques to reduce ost file size
Instead of increasing the OST file size limit, it is desirable to prevent the OST file from becoming so large. Outlook offers the following methods and settings for this;
- Ost files contain compressed mailbox data.
Starting with Outlook 2013, some of the contents of the OST file are actually compressed to accommodate more content while keeping the OST file up to 30% smaller than the actual mailbox. - Set the synchronization slider.
Another feature that Outlook 2013 and later versions have to keep the OST file size smaller is the synchronization slider. It allows you to set the length of time emails should remain cached in the OST file on your local computer.
File-> Account Settings-> Account Settings…-> double click on your Exchange account-> move the Sync Slider below: Download email for the past
- Move mailbox data to the online archive.
When you have an Exchange mailbox with an online archive, for example with an Office 365 E3 license, you can move the contents of your mailbox to your online mailbox to reduce the size of your mailbox and thus reduce the size of the ost file. This is because the online archive is not cached locally. - Do not cache shared folders and/or public folder favorites.
When you access shared folders, such as a shared mailbox or a manager’s or colleague’s mailbox, or configured favorite public folders, Outlook may also cache them. You can disable this feature via;
File-> Account Settings-> Account Settings…-> double click on your Exchange account-> button: More Settings-> tab: Advanced-> disable: Download shared folders
- Reduce the number of shared mailboxes.
You can set up the mailbox as an additional account instead of a shared mailbox. This will give the mailbox its own ost file.
