{"id":502,"date":"2026-10-08T18:47:42","date_gmt":"2026-10-08T18:47:42","guid":{"rendered":"https:\/\/www.brevisoft.com\/blog\/?p=502"},"modified":"2026-10-08T18:48:21","modified_gmt":"2026-10-08T18:48:21","slug":"how-to-fix-outlook-ost-file-too-large-error","status":"publish","type":"post","link":"https:\/\/www.brevisoft.com\/blog\/how-to-fix-outlook-ost-file-too-large-error\/","title":{"rendered":"How to Fix the Outlook OST File Too Large Error? Complete Guide"},"content":{"rendered":"
Outlook shows an error message saying “The OST file has reached its maximum size”<\/strong> when your offline mailbox file becomes too big. The default limit is generally 50 GB; after the file has reached this size, Outlook might stop synchronizing, and new emails could fail to arrive. The good news is that several methods can fix the issue. You can delete large emails, compact the OST file, decrease the amount of mail stored offline, archive older messages, or raise the file limit by using the Windows Registry. In the case where the OST file is too large or damaged, the BreviSoft OST to PST Converter tool<\/strong><\/a> can recover the mailbox data and save it in PST format for use with Outlook, Thunderbird, and other email clients supported by the program.<\/div>\n

Why does the OST file become so large?<\/h2>\n

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.<\/p>\n

As time goes on, the file may become quite large.<\/p>\n

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.<\/p>\n

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.<\/p>\n

Fix 1: Delete Unnecessary Emails and Attachments<\/h3>\n

Begin by getting rid of the ones that are not needed.<\/p>\n

In Outlook, search for:<\/p>\n

size:>10MB<\/p>\n

It enables you to locate large messages rapidly. Look over the results and get rid of old attachments, newsletters, and other unnecessary emails.<\/p>\n

Don’t stop there.<\/p>\n

Empty Deleted Items<\/strong> and Junk Email, too<\/strong>. If your mailbox has a Recoverable Items folder with unnecessary data, clean it up according to your organization’s retention policies.<\/p>\n

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.<\/p>\n

Which is why you should compact it then.<\/p>\n

Fix 2: Compact the OST File<\/h3>\n

Compacting removes unused space left after deleting large amounts of data.<\/p>\n

Open Outlook and go to:<\/p>\n

File > Account Settings > Account Settings > Data Files<\/strong><\/p>\n

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<\/strong>.<\/p>\n

Select Compact Now<\/strong>.<\/p>\n

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.<\/p>\n

As a result of a major cleanup, compaction is able to regain a useful amount of disk space.<\/p>\n

Fix 3: Reduce the Mail Stored Offline<\/h3>\n

It might be that you don’t need several years’ worth of email stored on your computer.<\/p>\n

With a Microsoft 365 or Exchange account, Outlook enables you to determine how much mail is stored offline.<\/p>\n

Go to:<\/p>\n

File > Account Settings > Account Settings<\/strong><\/p>\n

Select your Exchange or Microsoft 365 account and click Change<\/strong>. Find the Mail to keep offline<\/strong> setting and reduce it to about 3 <\/strong>or 6 months<\/strong>.<\/p>\n

Outlook will then rebuild or update the OST file, storing less mailbox data locally.<\/p>\n

Your older emails aren’t necessarily deleted. They remain on the server, so you can access them online when needed.<\/p>\n

One of the simplest methods for keeping an OST file under control is this.<\/p>\n

Fix 4: Archive Older Emails to a PST File<\/h3>\n

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.<\/p>\n

In Outlook, go to:<\/p>\n

File > Info > Tools > Clean Up Old Items > Archive this folder and all Subfolders<\/strong><\/p>\n

Select a cutoff date; for instance, you might archive messages that are more than two years old.<\/p>\n

Chose a location for the PST file and then begin the archive process.<\/p>\n

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.<\/p>\n

Keep the PST file somewhere safe. Ideally, keep a backup instead of storing the only copy on your desktop or laptop.<\/p>\n

Fix 5: Increase the OST Size Limit in the Registry<\/h3>\n

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.<\/p>\n

Use this method carefully.<\/p>\n

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.<\/p>\n

To press Windows plus R<\/strong>, type regedit and then press Enter.<\/p>\n

Go to:<\/p>\n

HKEY_CURRENT_USER\\Software\\Microsoft\\Office\\16.0\\Outlook\\PST<\/p>\n

Create these two DWORD (32-bit) Values<\/strong> if they don’t already exist:<\/p>\n