Disable mail for a domain

[kilburn]

The procedure explained by joximu is to tell postfix that it is not the recipient of the domain's e-mails, yet it will not magically change the DNS MX records to your external email server.

You'll have to do this modification manually, either through the custom DNS manager or by directly modifying the bind zones in /etc/ispcp/bind/working/domain.tld.db *and* /var/cache/bind/domain.tld.db. Don't forget to update the serial number.