Whenever an object associated to a newsletter is modified, a new version is created and both the old version and the new one enter the send queue, and are indeed sent.

It is impossible to remove a newsletter from the queue.
If the associated object is removed, the newsletter is still sent, as an empty mail.

This makes the whole extensione pretty unusable, since a user can not undo a mistake.

Steps to reproduce

A newsletter type is created.
A user subscribes to that newsletter.
A newsletter is created, using a valid content class named "Newsletter issue".
The object of class "Newsletter issue" is then modified and re-published.

The build_list and send_newsletter cronjobs are executed.

The subscribed user receives two email messages