Domain Admins are not what I'm asking for...
A domain admin is a MailBox user that has rights on his domain (and on that only)
What I'm asking for is a completely different type of user.
Your answer seems wrong to me.
EDIT: I know that I can use API to do that, BUT I was suggesting to add it as a standard SmarterMail function in the standard web administration portal
Gabriele Maoret - Head of SysAdmins 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)