Sometimes while opening Outlook 2016 users receive an error on the computer screen which states “ost is not an outlook data file”.

The first thing the user will do is re-start the outlook. While another approach is users can start Outlook in safe mode. It can be done by clicking start and Outlook.exe along with /safe parameter.

One technique is deleting the present outlook profile and re-create the profile as doing this will create a new OST file.


What the problem with that?

Many users don’t sync with Exchange or Office 365 and for example, during working offline, users prepare a number of emails and other work, and all of a sudden you end up re-creating the OST file with a new profile, all that offline work will be lost.


Reasons behind ‘Exchange.ost is not an Outlook Data File’ issue:

Outlook OST file is an excellent thing but the issue with it is that the file is not portable and as soon as you use it in another instance it will not open. MS outlook is not able to open an OST file as you do with a PST file. OST is a copy of your live mailbox and sync of it, therefore it’s not portable or being able to be opened by another instance.

Outlook PST is a personal file and can be easily ported and attached to another mailbox. The OST file is used to sync your mailbox to enable the user to work offline even if you don’t have a connection to your Exchange being on cloud or on-premise and when a connection is detected, it will sync all changes to the mailbox.

That particular problem occurs when Outlook’s OST file is not being trusted by the Exchange server as it’s seen as not recognized as in the format of the file. Because of Outlook’s advanced security, it will not allow you to open the OST file and the application will not load. 


To fix this problem one can also try the below:

Ensure one thing that you are online and your Exchange server is reachable from the computer.

  • Firstly, open Control Panel, find and open the Mail button.
  • Now click Show Profiles and click on the active Outlook Profile.
  • After that Click on Account Settings, select your email address, and untick the Use Cached Exchange Mode.

Now open MS Outlook and see that it loads with no issues. Unfortunately, the changes will not be visible since you will access the mailbox directly from your server.

  • Terminate Outlook.
  • Navigate Control Panel again an enable Use Cached Exchange Mode again.

Again try to launch Outlook and see if it opens and the problem goes away. But if the OST file is corrupt then there is no way to attach or open it in Outlook to export any changes.

Hence the only 2 methods that you could use is to either delete the OST, start with a new profile and accepting the loss of the changes you made or look at alternative applications to export any data items from the OST or convert OST  to PST format thus making it usable in another Outlook.

SysInfoTools OST to PST Converter – The software is very easy to use but really effective in the conversion. After installation, open the application and there is no requirement to look for the exact location of the OST file as the application has a find button. As the scan process finishes, the OST to PST converter will present with an Outlook-like interface with all my emails, calendar, tasks, contacts, and journals. 


Time to Sum up:

Through this blog, I tried to give the best solution to fix why Outlook shows exchange.ost is not an outlook data file message. I hope you understood the described techniques.