Not that I am campaigning against Topicbox, but I think it's worth making clear that just because your email is hosted on Exchange, doesn't mean your mailing list solution needs to be.
You can run, and I have in fact run, mailinglists on top of Mailman / Debian / Postfix / Nginx, in a heterogenous environment where the mailservers were Exchange (and some shitty hosted Exchange run by morons to boot). You can do it without even reconfiguring Exchange if you are willing to put all the lists in a subdomain with its own MX record.
No reason why a hosted solution couldn't work this way as well. If you don't want to have your mailinglists in a subdomain, then you would need mailserver configuration to set up the aliases correctly, and that creates a certain amount of maintenance hassle every time you create a new list, but I've never been convinced that's a worthwhile tradeoff to make. Making mailinglists instantly discernible from humans' addresses isn't necessarily bad.
