Sorry Michael to resurrect this post from a while ago. I had a look at the time, and again today, the ‘Email Users’ plug-in and ‘Post Notification’.
Neither have the ability to customise a list / category of users. They only allow for users groups from the start WordPress format (subscriber / editor / admin, etc.).
What I am looking to have when making a new post, is the option to either email ‘all’ or ‘customers’ or ‘non-customers’ or some other category which maybe needed in the future.
Any suggestions would be welcome and explored.