Using Subscription Management in Emails
Add Subscription Management to Email Messages
If you haven't set up Subscription Management, see that article for steps.
Once you have your established your categories, you need to categorize all new email messages going forward using the Category option.
In the Address step for any new message, draft, or template, you will see the Subscription Category option near the top of the dialog window. If you do not see the Category option, ensure that the Subscription Management feature is turned on.