social/mass_mailing_custom_unsubscribe/readme/USAGE.md

Once configured:

1. Go to Email Marketing > Mailings > Create.
2. Edit your mass mailing at wish, but remember to add a snippet from Footers, so people have an Unsubscribe link.
3. Send it.
4. If somebody gets unsubscribed, you will see logs about that under Email Marketing > Unsubscriptions.