What sort of tasks can you not do as a domain admin? I know for example the bulk provisioning in the web interface is very limited. Also you cannot add aliases in bulk as a domain admin in the web interface. Quite a lot of the Zimbra functionality is only available from command line tools that are run locally on the mail server I believe and require some Unix skills to use.
Additionally I do not know if the domain admin can run command line tools against his or her domain. If this is a feature offered by Zimbra perhpas Link2ICT could give SSH access to tecchies in schools.
Zimbra CLI Commands
I think the limitations of the delegated bulk provisioning are due to Zimbra's lack of functionality in the area of delegation of administrative roles rather than a decision by Link2ICT
Here is a definition of Global and Domain administrators
Administration Console - Zimbra :: Wiki

