by default, mailing list archives are kept in two directories:

/var/lib/mailman/archives/private/[[list name]]
/var/lib/mailman/archives/private/[[list name]].mbox

ideally, these would be symlinked to /home/[[domain.com]]/mailman/[[list name]] for backup/restore purposes as well as quota measurements. the domain account should own the folders, with the list user being added to the domain group similar to the webserver user. if this is not possible, then the mailman folder could be owned by the domain account user, but group ownership set to the 'list' user.

* based on my current setup on Ubuntu 6.06.1 LTS