Outlook data file .ost cannot be opened and unable to read offline email messages after switching from Office 365 to Office 2021

Question

I recently installed Office 2021 Plus (I had Office 365 license that expired). Prior to the installation of Office 2021, I had offline email messages in Outlook 365 that I could access without signing in to the Exchange account with a .ost file. I still have cached exchange mode checked after switching from Office 365 to Office 2021. After installation of Office 2021, I can no longer access the offline email messages in Outlook 2021 with the .ost file from Outlook 365. See below the error message that I received when I clicked on the Exchange Account trying to expand the email folders.

I don't need to synch with the Microsoft Exchange to retrieve new/updated emails, but I do need to retrieve certain emails that are in the .ost file. Based on the information at Microsoft Support, there are no .pst files associated with the Outlook 365, only .ost file.

I have tried repairing the both .ost files associated with my Microsoft Exchange and Yahoo account data files (.ost) using the ScanPST.exe and Outlook still could not be launched even after both .ost files were repaired. Disabling all add-ins didn't help either.

Why was I able to read/access the offline messages in Outlook 365 but could no longer see them? How can I retrieve the the offline email messages that are in the .ost file without signing in to the Exchange account just like when I was in Outlook 365?

Do you guys have any suggestions? Greatly appreciate your insights.

Answer 1

The OST file highly encrypted, doesn't allow to import the Outlook directly. When the account has been deleted from the server on that case It turned into the Orphaned state. Futher you cannot access it without converting it to PST file.

But, first you have to assure, accessing the OST file should be legitimate.

Answer 2

Hi @who ,

Thanks for your reply!

According to your description, i'm afraid that your Outlook profile is damaged. It's suggested that you could create a new profile (Control Panel>Mail>Show Profiles>add) and add the account related to the .ost file.

I notice that you don't want to continue to use this account but only need the history messages. It's suggested that you could export the data to a .pst file and then remove this account in Outlook. In this way, when you want to retrieve the the offline email messages, you just need to open the .pst file in Outlook.

If you are unable to add the account related to the .ost file, i'm afraid that the messages could not be available.

Hope the above help!

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