When you use Microsoft Outlook in cached mode, what I always recommend, and you add additional mailboxes to your outlook profile, you will notice that the OST file will grow. Outlook will download the mailbox items (mails, calendar entries, contacts etc.), and store them in the OST file. This is the default behaviour since Microsoft Outlook 2010. If you want to disable this behaviour, you have two options:
- Edit the registry
- Use a group policy object (GPO)
Edit the Windows registry
The easiest way is to use a reg file. Copy this text into a file and save it as disablecachedmode.reg. Then double click the file and confirm, that you want to import the registry file.
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Cached Mode] "CacheOthersMail"=dword:00000000
Please note the version number after “Office”.
Make sure that you use the appropriate version number for your Outlook! Otherwise this setting is applied, but not working.
If you want to apply this setting on a bunch of clients, you should use a GPO. Before you can use a GPO, you have to install the necessary template files for Microsoft Office/ Outlook. These ADMX files are part of the “Office 2013 Administrative Template files (ADMX/ADML)” or “Office 2016 Administrative Template files (ADMX/ADML)” package.
Copy the Outlk16.admx or Outlk15.admx files to the PolicyDefinitions folder (either C:\Windows or Central Store), and the Outlk16.adml or Outlk15.adml to the corresponding language folder.
Then you can create a new GPO. The desired setting can be found under User Configuration >> Administrative Templates >> Microsoft Outlook 201x >> Outlook Options >> Delegates >> Disable shared mail folder caching. Set this to “enable” and apply the GPO.
Still using Microsoft Outlook 2010?
Please note, that the GPO template for Microsoft Outlook 2010 doesn’t contain the necessary setting that controls this functionality!