MRD sends short email highlights when something notable happens — a strong run, a race recap, a fitness milestone. You’ll get at most one per run, so they should stay rare and worth opening.
To turn them off (or back on): use the Notification emails toggle on the 
Manage page, or tap unsubscribe at the bottom of any highlight email you’ve received.
These emails are described in the guide as “a preview of the in-app notifications, delivered by email for now” — so if in-app notifications land as a separate feature later, expect this toggle’s scope to change; nothing about that is confirmed here, just flagging the guide’s own wording.
Full explanation: Notification Emails in the guide.