You may need these to configure your 3rd party antispam software/appliance. There were lots of articles on the internet with complicated solutions, which did not work for me. Here a Powershell one-liner for Exchange management shell.

Having read some useful topics on the internet, I decided to try non-cached Exchange mode in Outlook. After disabling caching for my account, it still kept the OST file, even after forced deletion. Then I noticed some offline folders were being kept.