List Metrics
From Harmony Lists KB
While Mailman is a traditional mailing list platform not designed with the sort of tracking provided by dedicated marketing programs, the Affinity web interface does offer some performance metrics.
After logging into your list admin, you will see a "List Metrics" section just below your list's name. The metrics shown are from a rolling 14-day window.
The stats are grouped into three sets - Bounces / Messages, Subscribed / Unsubscribed, and Total Members. Clicking the entries below the chart changes the view.