Bruce Scherzinger has investigated a number of existing Joomla! extensions and ended up writing his own "Content Notifier" Joomla! plugin to do what he needed with regard to generating Newsletters from Joomla! articles and integrating with external mailing systems such as Constant Contact, Mailman, MailChimp, or Dada Mail. It actually works with any email address or list of addresses, but it has no front end user interface at all.
He demonstrated how the XML and php he wrote work. He also demonstrated the plugin, only to find some sort of issue in onContentAfterSave-triggered plugins when saving articles in the site back end. He found the issue after the meeting adjourned and began investigating the problem by posting to the joomla.org forums.
The attendees asked a lot of very good questions that had largely to do with site member choices and security. The idea behind this plugin is that you can configure it, for instance, to associate a single article category with a specific (external) email list. Then, user subscription to the article category is done by the user selecting to be subscribed to that email list. With Bruce's sites, that is done using his Dada Mail Subscriptions CB plugin, which integrates the Dada Mail email list system with Joomla (demonstrated at a prior NovaJUG meeting). Regarding security, Dorothy asked if it was possible for any user to subscribe any other user's email address to one of the Dada Mail lists. Yes, that's possible as it is possible using Dada Mail's native web interface or any other email list system if you don't set it up properly. Bruce explained that to a large extent he has relied on the fact that he knows and trusts all his site users (HOA and class reunion sites). But he recently purchased a Community Builder subscription and is investigating the CB ReConfirm Email plugin as a new integration with his Dada Mail Subscriptions plugin. Assuming this integration works, it would require any email address entered by a user in their profile to be confirmed before the address is actually saved and subscribed to the list. The Dada Mail Subscriptions plugin would have to take that into account and trigger off the new CB event before actually subscribing the new address to the list. Maybe a topic for a future JUG meeting?
Some relevant links not shown during the meeting:
The Content Notifier plugin is avaliable for download from Bruce's Github repository as are his other extensions that are being kept current.
Follow-up regarding the issue with the plug-in crashing for back-end article saves:
Turns out this is a known limitation that is scheduled for resolution in Joomla! 3.9. Here's a link to the joomla.org forum discussion on the subject. Looks like the person answering the mail on this issue appreciated the details Bruce provided.