Forwarding mail if unser unknown possible?

[truster]

Hello, I have setup a server with this great howto: http://www.howtoforge.com/perfect-se...nx-ispconfig-3

Everything works fine, but i have the following situation:

i use the e-mail domain from my isp (@aon.at) with getmail. All outgoing mail goes to the smtp server from the isp. if i send a e-mail to an address, that is not in my virtual mailbox table, for example: xyz@aon.at, i get the following message:

5.1.1 <xyz@aon.at>: Recipient address rejected: User unknown in virtual mailbox table.

send to all other domains works.

is it possible to forward those mail to an external relay host?
In Exchange 2003 i had to delegate the domain as non-authoritative and added a transport for aon.at. All unresolved adresses would be forwarded to the specified host.

Is this possible with ISPConfig3?

Any Help would be greatly appreciated.

Thanks.