Is there a wordpress listserv or email group plugin?

We’re currently developing this for a client. It turned out to have pretty complex needs. For example, as the group grows, the number of emails being sent becomes huge (i.e. every email response goes to hundreds of people), so we are running that part of the service through SendGrid. We want everything to run through the front end, so we had to develop that. There’s also all sorts of permissions, and automatic emails that need to have a space for configuration. Emails need to be stored in some kind of thread in an archive, so we had to work that out. We’re almost finished development, and then we have to see how it runs. We are considering turning it into a plugin/theme (it needs both components), but that would take a while to release. It’s been a very interesting project.