As has been passed around here and there over the past few weeks we'd like to de-Google ourselves as much as possible. Presently Google Groups is used for mass emailing as you know.
It would be nice to send out responsive html emails bi-monthly.
Anything we were to adopt would need to be OS ideally, and products like MailChimp cost a lot of money. MailMan has been suggested but there have been bad reports about the time administering non-spam justification can eat up.
Any ideas you have I'd love to hear them.
Incidental: http://www.sophimail.com/ / https://github.com/sophimail/webadmin - "looks like it only supports Postfix with virtual users (i.e. stored in a database) and PostfixAdmin installed. Still pretty neat, just be aware that you're going to have to layer a PHP admin interface on top of Postfix."