The issue still persists, and I've heard nothing from eukhost for a few days.
The ADMIN setting means that the mailbox user could log in to the MaiEnable Admin
website hosting and create groups and lists. There's a bug with the HC, in that if you Edit a Mailbox, the setting sticks at ADMIN. I submitted a ticket a while back, but if the speed of the migration is anything to go by, it will be a long time before a resolution!