Campus Mailing Lists, List Owners and Moderators
A current member of @illinois.edu must be an owner and be the sponsor of a Campus Mailing List.
Are there restrictions for mailing list owner/moderator?
That said, the owner can designate anyone to assist in managing and moderating the mail list. There is no domain restriction for an owner or moderator so members from other organizations can use their @where.ever email address.
Details about adding or updating the owners or moderators list can be found in the Campus Mailing List User Guide, https://wiki.illinois.edu/wiki/display/maillists/Adding+or+Updating+List+Owners+or+Moderators, or KB article Campus Mailing Lists, Add moderators or owners.