Hi,
Has this feature ever worked before?
Does "highlight" work in other programs, such as Word, Excel?
Which highlight color did you set? Please make sure you set it to a bright color (such as red, yellow or green) so that it won't mix with the background color of your Outlook messages pane.
In addition, please make sure you have enabled Windows Search Service on Windows Server 2012. If not, you can follow these steps to do it:
1. Start Server Manager.
2. Click Manage, and then click Add Roles and Features.
3. On the Before You Begin page, click Next.
4. On the Installation Type page, select Role-based or Feature-based Installation, and then click
Next.
5. On the Server Selection page, select the server or virtual hard disk on which to install Windows Search Service.
6. On the Features page, select Windows Search Service, and then click
Next
7. On the Confirmation page, verify that Windows Search Service is listed, and then click
Install.
If Windows Search Service has already been enabled, please also try rebuilding the index to check the issue. It's reported that this issue may happen due to corrupted index file. File > Options > Search > Indexing Options > Advanced > Rebuild.
Hope this helps.
Regards,
Steve Fan
TechNet Community Support