Among the +Create New buttons in the wiki, you should see an option called Start New Feed.
You would name your feed, "Newsletters" or something like that, and set it as a Global Feed (vs a Personal --or private-- Feed). In the ordering options, you want your feed to be ordered by when content was added to the feed. Put a link to this feed in a location your users will see it.
Whenever you want to send a new newsletter, you would go to the feed, and select Add Entry > Add New Page. The page content should be your newsletter. When you save the page, it should notify users who are subscribed/watching the feed that a new newsletter was added.
Or, you can have an area called "Newsletters" and follow a similar process (where the users would watch the area), but the ordering options for feeds are better suited to this.