When using Mail Merge to send emails, EmailEngine allows specifying an additional property,
listId. If this value is set, then EmailEngine treats this message as a virtual mailing list email.
EmailEngine does not have "real" mailing list capabilities – there are no contact lists or segmentation rules, and there is no recorded history. Almost everything you'd expect from a mailing list manager is missing from EmailEngine. Virtual mailing lists provide only a single feature – list-unsubscribe.
Sometimes, for example, for legal reasons, it might be necessary to offer an unsubscribe option to the recipients. Building all the required components only to support unsubscribing might be tedious, so EmailEngine provides it itself.