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Why does the OST file become so large?
To enable offline access, Outlook creates an OST file which contains a local copy of your mailbox. This file holds the emails, attachments, calendar data, contacts, and all other mailbox information.
As time goes on, the file may become quite large.
Large attachments are often the main reason. Old emails also add up, especially when you have years of messages. Deleted Items and Junk Email can make it worse if you never clean them out.
A mailbox doesn’t need to seem busy in order for its OST file to grow. It is possible for a few large attachments each week to accumulate over a number of years.
Fix 1: Delete Unnecessary Emails and Attachments
Begin by getting rid of the ones that are not needed.
In Outlook, search for:
size:>10MB
It enables you to locate large messages rapidly. Look over the results and get rid of old attachments, newsletters, and other unnecessary emails.
Don’t stop there.
Empty Deleted Items and Junk Email, too. If your mailbox has a Recoverable Items folder with unnecessary data, clean it up according to your organization’s retention policies.
There is one important point to note. Removing emails does not immediately cause the physical size of the OST file to decrease since Outlook can leave unused space within the file.
Which is why you should compact it then.
Fix 2: Compact the OST File
Compacting removes unused space left after deleting large amounts of data.
Open Outlook and go to:
File > Account Settings > Account Settings > Data Files
Choose the appropriate Outlook data file and then select Settings. In some versions of Outlook, the compact option can be found under Advanced > Outlook Data File Settings.
Select Compact Now.
Keep your patience while carrying out this procedure, since a big OST file may take a while to be compacted. If Outlook is busy or the option is not available, then close it and reopen it before attempting it again.
As a result of a major cleanup, compaction is able to regain a useful amount of disk space.
Fix 3: Reduce the Mail Stored Offline
It might be that you don’t need several years’ worth of email stored on your computer.
With a Microsoft 365 or Exchange account, Outlook enables you to determine how much mail is stored offline.
Go to:
File > Account Settings > Account Settings
Select your Exchange or Microsoft 365 account and click Change. Find the Mail to keep offline setting and reduce it to about 3 or 6 months.
Outlook will then rebuild or update the OST file, storing less mailbox data locally.
Your older emails aren’t necessarily deleted. They remain on the server, so you can access them online when needed.
One of the simplest methods for keeping an OST file under control is this.
Fix 4: Archive Older Emails to a PST File
If you have a mailbox containing messages from year after year that you seldom open, then it makes sense to transfer the older mail into a PST archive.
In Outlook, go to:
File > Info > Tools > Clean Up Old Items > Archive this folder and all Subfolders
Select a cutoff date; for instance, you might archive messages that are more than two years old.
Chose a location for the PST file and then begin the archive process.
The older messages move out of the active mailbox and into the PST. This can help reduce the amount of data Outlook needs to keep in the OST.
Keep the PST file somewhere safe. Ideally, keep a backup instead of storing the only copy on your desktop or laptop.
Fix 5: Increase the OST Size Limit in the Registry
If none of the earlier methods manage to solve the problem, then you can increase the limit of the Outlook data file by means of the Windows Registry.
Use this method carefully.
Before carrying out any changes to the Registry, it is necessary to back it up or to export the relevant key. Making a wrong edit to the Registry may have an effect on Outlook or on other Windows applications.
To press Windows plus R, type regedit and then press Enter.
Go to:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
Create these two DWORD (32-bit) Values if they don’t already exist:
- MaxLargeFileSize: 81920
- WarnLargeFileSize: 77824
The values are given in MB, meaning that 81920 corresponds to a limit of 80 GB.
Once you’ve made the changes, you should restart Outlook.
While raising the limit will give you more space, it should not be considered a permanent fix since very large OST files can take a longer time to process and might impact Outlook’s performance. It is generally better to keep the mailbox under control.
What if the OST file won’t open?
Sometimes the OST file is severely oversized or damaged. Outlook may then refuse to open it, which means the normal cleanup methods aren’t available.
A rather common situation is this: when the original Exchange or Microsoft 365 account is no longer accessible, the new Outlook profile has nothing to synchronize from the old mailbox.
That’s where a specialized recovery and conversion program can be of assistance.
The BreviSoft OST to PST Converter is able to read an OST file directly and extract the mailbox data; you won’t need to depend on the original Outlook profile or on an active Exchange account in order to access the file.
You have the option of previewing the mailbox folders and items before converting them, and you can choose the data that you want to export, use filtering based on date, and divide large PST output files into smaller sections.
For example, if you have an OST file of 55 GB that Outlook can no longer open, rather than having to create new profiles over and over again, you can load the OST file into the BreviSoft OST to PST Converter, check the mailbox data that is available, and then export it to smaller PST files.
If you are moving away from Outlook, the data that has been extracted can also be saved in formats that are supported for use with other email programs, such as Thunderbird.
Conclusion
The fact that you have an oversized OST file does not necessarily indicate that your mailbox is damaged.
Start with the simple fixes. Remove large and unnecessary emails, empty unwanted folders, compact the OST file, and reduce the amount of mail stored offline. If you have years of old messages, archive them to a PST to keep the active mailbox smaller.
Increasing the Registry limit is another option, but use it carefully. It gives the file more room but doesn’t fix why it became so large.
If Outlook can’t open the OST file at all, the situation is different. In that case, BreviSoft OST to PST Converter can help extract the mailbox data and save it in a usable format before you continue with your Outlook migration or recovery.
