Unfortunately, this isn't currently possible (it would be nice if it were implemented in the future...).
What you can do right now is: set a limit on the total space that can be occupied by the domain (for example, I set it to the sum of the space of the "sold" mailboxes).
This way, you'll give domain administrators the freedom to set their mailboxes as large as they want (even "infinite," if they want...), but the actual space occupied by the SUM of all the domain's mailboxes will always be limited by the maximum space for the domain itself.
Gabriele Maoret - Head of SysAdmins and CISO at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)