By default, Outlook sets the category on emails you select (by clicking on the Categories column in your Inbox) as Red Category. If you often are categorizing your email as something other than Red, you can change the default Quick Click setting, follow these steps:
1. Right-click any Category Icon in your Inbox and select Set Quick Click (or you can click the Categorize icon on your standard toolbar and select Set Quick Click).
2. From the drop-down, select the desired Category setting.
3. Click OK.
Now when you click the Category Icon on an email, it will use your new setting as the default.
