Summary: OST file has reached the maximum size is an error message that appears when the size of OST reaches its configured file size or threshold limit. In this guide, we have shared a few fixes and solutions that can help you resolve the OST file size issue in Windows Outlook.
OST or Offline Storage Table is a replica of the user mailbox on the Exchange Server. The file is stored on the system locally and syncs the changes made to Outlook with the user mailbox on the server. When the size of OST increases and reaches its threshold or file size limit configured on the system, the error OST file has reached the maximum size may appear.
The error may also appear when the OST file size reaches the configured mailbox size limit on the Exchange Server. In such cases, the changes you make are not saved, or emails you send are not sent. Similarly, you stop receiving new emails and face several other issues until you resolve the error.
One way to resolve the error is to ask the Exchange admin to increase the mailbox size limit. Once the mailbox size is increased, it should probably fix the error. However, the error will soon appear when OST reaches the new threshold limit.
But if you control the OST file size, there is no need to increase the mailbox size limit. Below, we have shared a few methods that will help you reduce the size of the OST file and resolve this issue.
Outlook has an option to compact the OST and PST files to reduce their size. The steps are as follows:
This will compress the OST file to an extent and may resolve the issue temporarily.
To fix the issue for a long time, To reduce the OST file size, you can choose to clean up your mailbox. However, this method requires you to delete older emails. The steps are as follows:
Further, you can look for large emails, download all the attachments, and then remove the attachments from emails to reduce the OST file size.
Shared Folders can also quickly increase the OST file size and cause Outlook performance issues. To resolve this, disable Shared Folders synchronization by following these steps:
Outlook provides an option to limit the synchronization, enabling you to choose how much data or mail items you want to synchronize to the local OST file. The setting is called Mail to keep offline. It’s a slider in the Server Settings dialog box available in Outlook 2010, 2013, 2016, 2019, and 2021 that you can adjust to change how much data to keep offline. This can help you significantly reduce the OST file size as the setting is applied to both the user mailbox and shared folders.
The steps are as follows:
You can choose between 3 days to 5 years or All. The higher range you select, the more Outlook will synchronize, resulting in a large OST file size. For example, if the slider is at 2 years or more, move it to 1 year or 6 months, and check the OST file size after a while. You will notice that the OST file size has reduced significantly.
The only trade-off is that you may not get the emails in the result that are older than the range you’ve selected in Mail to keep offline settings during the search.
If you don’t want to delete older emails or remove attachments from emails, there’s an alternative way, where you can use the registry to reconfigure the OST file size limits by changing the registry key’s entries and values. There are four registry entries that you can edit and set the maximum size to which OST can grow,
This lets you set the Unicode formatted OST file size limit to a maximum of 50 GB for Outlook 2010 and later or 20 GB for Outlook 2007 and 2003
Refers to the ANSI formatted OST file size limit to a maximum of 2 GB (Outlook 2000)
The following are the default and valid range of ANSI and Unicode formatted OST that you can change.
For Outlook 2019, 2016, 2013 and 2010
|Name||Valid Data Range||Default|
|MaxFileSize||0x001F4400 – 0x7C004400||0×7BB04400 2,075,149,312(1.933 GB)|
|WarnFileSize||0×00042400 –0×7C004400||0×74404400 1,950,368,768 (1.816 GB)|
|MaxLargeFileSize||0×00000001 –0×0000C800||0×0000C800 51,200 (50 GB)|
|WarnLargeFileSize||0×00000000 –0×0000BE00||0×0000BE00 48,640 (47.5 GB)|
Refer to this guide on the Microsoft Support page to set the new or reconfigure the OST file size limit.
Instead of deleting older emails, you can move them to a local PST or personal folder file. This can help reduce OST size significantly while keeping all emails intact. To do this, follow these steps,
You can also create a new folder in Outlook and move all emails with large attachments to this folder. Then by following the above steps, archive the folder. This will help you reduce the OST file size and keep large email items.
Also, now that the archive folder is created, you can move more items to this folder Also, now that the archive folder has been created, you can move more items to this folder manually to keep control of the OST file size. You can also create a new folder in Outlook and move all emails with large attachments to this folder.
However, it’s important for you to note that the Archives are saved in a personal folder file or PST format. And as you move more and more items in the archive folder, the PST file size will grow larger, which can lead to PST file corruption and data loss.
So, you can either create multiple archive folders to keep PST file size in control or use an OST to PST file converter, such as Stellar Converter for OST, to reduce the OST file size without the risk of PST or data corruption.
Stellar Converter for OST helps you convert the OST file into PST with 100% precision and integrity. After the OST file scan, you can select the mail items or email folders and save them to a PST file. In addition, it provides you with filter options that you can use to filter large or older emails and save them to a PST file. While saving PST, it allows compacting or splitting the PST file, which helps reduce the PST file size and avoids any corruption. Later, you can add the PST file to your Outlook account and access all the emails with attachments.
When OST reaches the maximum size, it prevents users from accessing their Outlook email account and restricts communication. To fix this, you can follow the suggested manual methods to reduce the OST file size. You can also convert OST into PST using a professional OST to PST converter tool. The software features option to split and compact the PST file. This helps keep PST size within the limit as PST files tend to get corrupt or damaged when the size grows larger.
For more help, leave a comment or reach our Self Help & Support page.
Ravi Singh is a Senior Writer at Stellar®. He is an expert Tech Explainer, IoT enthusiast, and a passionate nerd with over 7 years of experience in technical writing. He writes about Microsoft Exchange, Microsoft 365, Email Migration, Linux, Windows, Mac, DIY Tech, and Smart Home. Ravi spends most of his weekends working with IoT (DIY Smart Home) devices and playing Overwatch. He is also a solo traveler who loves hiking and exploring new trails.
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