All email lists hosted in the illinois.edu domain must be sponsored and owned by a current member of @illinois.edu.  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 or KB article: Campus Mailing Lists, Add moderators or owners.