Gert
How can i forward mail sent to admin@theirdomain.com to one account and administrative notices to another?
The easiest answer is to not use admin
A workaround would be to change the main primary domain of your server to something else (evena free domain name), then admin mail will go to admin@mainfreedomain
Then create a virtual domain which is your real domain name
Then setup a pseudonym for admin@realvirtualdomain to forward mail as usual