That email was for the New User of the Month badge. It is granted to two new users each month, based on how many posts they created and how many likes they received. The badge is enabled by default. You can confirm that it is enabled on your site by looking for the “New User of the Month” entry on your Admin / Badges page.
I believe it’s one of the few Discourse badges that generates a notification that will be sent by email. If the Discourse Automation plugin is installed on your site, you could use it to trigger Personal Messages to be sent when other types of user related events occur. For example, you can use it to trigger PMs for badges other than the New User of the Month badge. You can also use it trigger a PM when a user’s post is accepted as a solution, or when a user is added to a group. Possibly that would be a useful tool for increasing engagement.