Directory-based mailing list solution

I'm looking for a mailing list solution that meets a few criteria:

• Default membership based on Active Directory or LDAP groups/filters/whatever
• Users included by default can choose to opt-out
• Users not included by default can choose to opt-in
• Emails to the list are distributed to all other users (i.e. this is a group, not a 1-way broadcast)
• Email is only accepted from users on the list (though I can live without this)

The end-goal is to create a mailing list (or lists) where certain staff are included by default (e.g. all technical departments) but users can opt in or out at their leisure. I want to ensure that all new staff get automatically added, but also that edge cases (e.g. seconded non-technical staff or project managers in technical teams) can opt in or out (respectively) if they wish.

We use both Active Directory and LDAP and have them both heavily populated with useful data. We also use Exchange so if there's a way we can do it there I'd be happy to hear it.

Commercial solutions are definitely not excluded, but free (as in dollars) would be preferred. I don't mind a bit of manual glue scripting but something all-inclusive would be preferred. Linux or Windows OS both OK.

TIA!