Do you want the domain lists OR each individual email box across all domains ?
The first is easy as it is part of your administrator control panel when you log in and see all of the domains, on the far right is the disk usage for each domain.
To get each individual mailbox you click on that domain and then go to the "Accounts" tab
www.HawaiianHope.org - Providing technology services to non profit organizations, low income families, homeless shelters, clean and sober houses and prisoner reentry programs. Since 2015, We have refurbished over 11,000 Computers !