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Outlook 2013 synchronisation problem

Outlook 2013 can go into a spin and constantly upload duplicate messages, which it has cached locally, back onto Hermes. We have seen cases of many thousands of messages (including recently deleted ones) in a folder. Typically this affects the inbox, although we have seen cases where other folders are affected.

Last updated: September 2014

The root cause appears to be a corrupt offline Outlook Data File (the OST file). The problem appears to be caused by virus scanner plugins to Outlook.

Microsoft does not recommend trying to repair an offline Outlook Data File, so if you experience this problem you need to remove the corrupted file so that Outlook can create it again.

To do this:

  1. Find where the .ost file is from within Outlook by going to File -> Account Settings -> Account Settings
  2. Select the Data Files tab, then select the file and then click Open File Location. This opens the directory that contains this file in File Explorer (or Windows Explorer, depending on your version of Windows). For example, in Windows 8.1, for the user dpc99, it would be C:\Users\dpc99\AppData\Local\Microsoft\Outlook\ 
    (If you need to open File Explorer or Windows Explorer explicitly then, in Windows 8.1, search for File Explorer. In Windows 7 you can get to Windows Explorer by choosing the Start Menu -> All Programs -> Computer.)
  3. Close the Account Settings window but leave the File Explorer (or Windows Explorer) window open
  4. Exit Outlook (otherwise the final step will fail)
  5. Right click on the Outlook Data File in File Explorer (or Windows Explorer) then pick Delete to remove it.

The same procedure can also be found in the last section of the following page from Microsoft -

When you re-start Outlook the OST file will be recreated.