In the svn-version there is a new module called "domain". if you activate this module, your customer can only add a domain, you have added for them.
Means, if you said "d1.com" and "d2.com" are their domains, they can add a e-Mail-Domain, but only "d1.com" or "d2.com" - no more.
but! this is only experimental and only in the svn.
DO NOT USE SVN IN PRODUCTION ENVIRONMENTS!!!