Free Tips to Fix "OST File Not Recognized by Outlook" Error

Microsoft has now become one of the popular medium for the exchange of information from one to another organization. It comes up with so many features; one of them is its ability to synchronize with Exchange server. With its collaboration with MS Exchange, it creates OST file. There is a possibility that this file may get corrupt or damaged, which results an error "OST file not recognized by Outlook." In such situations, users look for a method to get out of this trouble, but it is not easy to find any suitable method. Moreover, it is very important to follow a relevant solution for the safety and security of the data. Therefore, in the upcoming part of the blog we will discuss some methods to resolve Outlook "OST file not recognized" error message. However, it is important to know the symptoms and causes behind the issue.


Whenever users try to open MS Outlook, they will receive this error message as mentioned below:

Outlook 2010 does not recognize OST file or Outlook 2013 does not recognize OST file depending upon the version of Outlook users are accessing.

You may also receive this message

"Cannot open your default email folder. The file xxx.ost is not an offline folder file."

Here, xxx.ost is the offline folder generated by Outlook at the time of working with MS Exchange mailbox offline. Users may be unfamiliar with the file due to its implicit creation.

Reasons behind "OST is Not an Offline Folder" Error

These are the two basic reasons behind this error:

1. The corruption in OST file. Once the OST file is damaged, MS Outlook cannot recognize it. Sometimes this error appears if OST file does not match mailbox size in Exchange Server. Therefore, in such situations Outlook reports this error.

2. With the synchronization with Exchange Server, Outlook creates OST file. Therefore, if MS Outlook unable to access Exchange mailbox due to some reasons or begin the synchronization with the mailbox of offline folders in the OST data file, then user will encounter this error.

Manual Solutions to Fix Outlook.ost is Not an Offline Folder File Error

For the rectification of the error and avoid data loss, you can follow these steps as mentioned below:

Firstly, locate the OST file, which is responsible for this error. It is easy to find the file with the help of the information that is mentioned in the error message. You can also find the file by using Search function available in Windows.

It is suggested to take the backup of Outlook OST file. This backup makes the data secure.

Another way is to delete the OST file, which causing the error. Or you may rename it as well.

In case the MS Exchange mailbox & email account are valid, then check the account settings in Microsoft Outlook. You need to check that Outlook can connect to MS Exchange server properly. After checking all the settings, you can start MS Outlook & receive or send the emails on the mailbox of Exchange. This will create a new OST file in an automatic manner & synchronize its data with the MS Exchange mailbox.

  • Firstly, you need to click on Start option, and then go to Control Panel.
  • Now, click Switch to Classic View in case you are accessing Windows XP/higher versions.
  • After this, double-click on Mail
  • Now, in the dialog box of Mail Setup, click Show Profiles
  • Choose the incorrect profile from the list & click on Remove to delete it. Repeat it until all the incorrect profiles have been deleted from the list.
  • After this, click on Add to make a new profile & add email accounts
  • Now, start MS Outlook, this will disappear the problem.

The Bottom Line

OST File Not Recognized by Outlook is an error that can occur at times in MS Outlook. As mentioned above, the reason behind the error is the corruption in OST files. Therefore, to repair OST files users can go for above-mentioned manual methods. In case the manual methods fail, they can go for a third party solution to repair and save OST Outlook to PST and fix the error.