Ok, are you wanting to be able to give admins of other domains the ability to administer their own email? Or is it that you want to limit a user to only one domain.
If it is the first one SME has a 'userpanel' contrib that will allow limited access to the 'Server-Manager' web page.
If it is the second question then their is also another contrib that makes users a memeber of only one domain.
As far as the first answer goes, I would also like to be able to do this. I would like to be able to have admins of other domains be able to administer their own mail rather than have me do it for them. I have asked this question before but their doesn't seem to be anything out there that does this. The userpanel I believe allows whoever you give access to be able to see also your own mail setup for your own domain, I do not like that idea as my brother is an idiot.

He can barley point and click and I don't want him muckin up my domain only his.
If I am wrong I would hope that someone else with a better understanding of this issue could enlighten me.
Jon