This plugin has been in sore need of updates for years, with many features requested by the community but little action by WPMU.
This plugin has gotten more attention lately due to the release of Hustle, where many of your users have asked about it. And, well, I guess I'm one of them.
What is your intention with the Newsletter Plugin? Are you going to update it or is it going to remain "fallow"?
If updating, what do you intend to do? Improve auto-response? Drip/Schedule Email? Change the creation interface? Improve analytics? Sending limit rules? Pro-Sites and Membership integration?
I'm happy to work with WPMU on improving this plugin -- we've used several mailer plugins on ThirdScribe over the years, including this one, and I have a lot of lessons learned to pass on. But, if worrying over this plugin is a waste of my time, please let me (and the community) know and mark it as something you will no longer be supporting.