I agree, wholeheartedly... however Mailman is not the answer, YET...
From the mailman docs:
One approach to handling virtual domains is to use a separate Mailman
installation for each virtual domain. (Currently, this is the only way
to have lists with the same name in different virtual domains handled by
the same machine.)
Jeff, if you can help make heads or tails out of the requirements and proper placement / syntax for Exim in the Mailman docs, I'm sure this would be a welcome addition to Installatron ... then we can add it to the skins and everyone could use this feature - IF the changes dont break Exim for everyone else (dont think they will, but ya never know)