Tracked To Dynamics 365 category added to user's mailbox

Users configured to use Server-Side Synchronization with Dynamics 365 may see a new category available in their Exchange mailbox and in Outlook. For example: If the user uses the Categorize option in Outlook on an item such as an email message or appointment, they see a new category called Tracked to Dynamics 365.

When viewing the Alerts area for a user's mailbox, you may see the following information level alert message:

"Outlook Category "Tracked To Dynamics 365" has been enabled for the mailbox [Mailbox Name] as the OrgDBOrgSetting is enabled. Use this Outlook category to track emails, appointments and tasks."

This experience is part of a feature which is now enabled by default:

Use Outlook category to track appointments and emails
https://docs.microsoft.com/dynamics365/customer-engagement/admin/use-outlook-category-track-appointments-emails 

You can disable this feature within the Power Platform Administration Center:

1. Access https://admin.powerplatform.microsoft.com as an administrator.
2. On the Environments tab, click ... next to the name of the environment and then click Settings. 
3. Click Email Tracking.
4. Locate the option People can use categories to track emails and appointments and switch the option to Off. 
5. Disabling this setting will remove the category Tracked to Dynamics 365 on all Exchange mailboxes of the Dynamics 365 for Customer Engagement organization which have server-side synchronization enabled in about 15 minutes.

As mentioned in the article above, it is also possible to disable this feature using the OrgDBOrgSetting named TrackCategorizedItems. The following information copied from the FAQ describes how to change this setting:

How do I remove category-based tracking through OrgDBOrgSetting? 
To disable the special Tracked to Dynamics 365 Outlook category, you need to enable the OrgDBOrgSetting in your Dynamics 365 for Customer Engagement apps organization. Dynamics 365 for Customer Engagement apps provides the OrgDBOrgSettings tool that gives administrators the ability to implement specific updates that were previously reserved for registry implementations.

1. Follow the instructions in this article for steps to extract the tool.
2. After extracting the tool, disable the OrgDBOrgSetting TrackCategorizedItems.
3. Disabling the OrgDBOrgSetting will remove the category Tracked to Dynamics 365 on all Exchange mailboxes of the Dynamics 365 for Customer Engagement organization which have server-side synchronization enabled in about 15 minutes.

Instead of using the command line option mentioned above, you can also use this tool to edit the OrgDBOrgSetting TrackCategorizedItems.