@xma: take a look at this link. It's the standard postfix configuration-documentation. It states how to set up a secondary mailserver, and it looks easy enough. I'm not entirely sure which postfix you need to edit, since I believe ISPConfig installs its own postfix, but I would assume you need to edit the files in the ispconfig subfolders. Please correct me if I'm wrong.
Example taken from above link:
When your system is SECONDARY MX host for a remote site this is all you need:
2 the.backed-up.domain.tld IN MX 100 your.machine.tld.
5 relay_domains = . . . the.backed-up.domain.tld
6 smtpd_recipient_restrictions =
7 permit_mynetworks reject_unauth_destination
9 # You must specify your NAT/proxy external address.
10 #proxy_interfaces = 18.104.22.168
12 relay_recipient_maps = hash:/etc/postfix/relay_recipients
17 . . .
When your system is PRIMARY MX host for a remote site you need the above, plus:
19 transport_maps = hash:/etc/postfix/transport
22 the.backed-up.domain.tld relay:[their.mail.host.tld]
* Do not list the.backed-up.domain.tld in mydestination.
* Do not list the.backed-up.domain.tld in virtual_alias_domains.
* Do not list the.backed-up.domain.tld in virtual_mailbox_domains.
Edit reason: added example from postfix.org site.