Primary use case
Send Messages on entity creation or update to Groups.
Group types are configurable as internal or external set of Contacts (example: Drupal Role, Drupal Group, Mailchimp List, CiviCRM Group).
Messages can be sent through a channel (example: website block, mail, PWA notification, …).
Mail relay is configurable per Group type (example: Drupal mail, Swiftmailer, Mailchimp/Mandrill, CiviCRM CiviMail, …).