@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.
http://www.postfix.org/STANDARD_CONF...ME.html#backup
Example taken from above link:
When your system is SECONDARY MX host for a remote site this is all you need:
1 DNS:
2 the.backed-up.domain.tld IN MX 100 your.machine.tld.
3
4 /etc/postfix/main.cf:
5 relay_domains = . . . the.backed-up.domain.tld
6 smtpd_recipient_restrictions =
7 permit_mynetworks reject_unauth_destination
8
9 # You must specify your NAT/proxy external address.
10 #proxy_interfaces = 1.2.3.4
11
12 relay_recipient_maps = hash:/etc/postfix/relay_recipients
13
14 /etc/postfix/relay_recipients:
15
user1@the.backed-up.domain.tld x
16
user2@the.backed-up.domain.tld x
17 . . .
When your system is PRIMARY MX host for a remote site you need the above, plus:
18 /etc/postfix/main.cf:
19 transport_maps = hash:/etc/postfix/transport
20
21 /etc/postfix/transport:
22 the.backed-up.domain.tld relay:[their.mail.host.tld]
Important notes:
* 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.
Best, Farao
Edit reason: added example from postfix.org site.