Is there a good self hosted e-mail reflector?

I would like to find an easy way for "Birds Of a Feather" (in my organization) to talk about their topics of choice.

However, a web-site IS NOT the answer. This needs to be something that just shows up in your inbox. The idea is, folks subscribe to these in house topics that they care about, and if someone posts, they get it. If it's too much volume, they can either go take themselves off the list or go digest only.

Yes, there is good old 'mailman' (http://www.list.org/) - but it's not as streamlined as what I'm imagining SHOULD exist.

The requirements are: * in house (sensitive stuff) * self administered (the users should be in control)

I feel like this sort of functionality is often a side-effect, but I'm not aware of anyone who's made this sort of the product the main course.