Situation:

I'm looking for mailing list software to help out (at least initially) a 
PR employee.  Here's the situation:

The companies current mail setup is standardized on using 
Outlook/Exchange.  Currently the PR employee maintains a distribution 
list in her Outlook Contacts.  The problem is, the list is having to be 
manually maintained and has grown larger than Outlook allows for a 
single distribution list.

I brought up the idea of using a mailing list system to handle this load.

There are two possible points where the mailing list could be run.  A 
company is hosting a frontend email server for so all inbound mail has 
to go through it before coming to our exchange server.  This is running 
qmail on Linux.  The other point this could be run at is on the web 
server which also has an instance of qmail installed on it.

 From what I've gathered, the requirements for the list software are:

All mail must look like it is coming from the PR employees email address
All mail must seem to be going to just the recipient (makes the email 
look like a personal mail)
Only certain email addresses should be able to send mail to the list 
(restricted to moderators, not just subscribers)

The problems:

If the software runs on the frontend email server, then it isn't under 
our control.

If the software runs on the web server, then it can't receive mail 
itself for the mailing list software since all mail for our domain goes 
through the remote frontend servers which are only set to forward to the 
Exchange system.

Also, due to limitations in Exchange (or so I'm led to believe) it won't 
send emails for an address in its domain out to a remote box, so the PR 
employee (and potentially other groups that might want to use a system 
like this) can't just create an email to the list and get it to work.

e.g. Exchange controls domain.com.  You can't send an email to 
maillist@domain.com and have it go to maillist@frontend.com.

Anyone have some pointers to help out with?  I'd prefer to stick to 
Free/Open software if that is possible.