URGENT: Where is the template or default settings for postfix/main.cf?!

[ottomotto]

Hello,

I want all mails to leave the server even thos that appear to be local, because the mail server is on a different IP.
For that I changed in /etc/postfix/main.cf from
virtual_transport = plesk_virtual
to
virtual_transport = relay

And after restart of postfix, it works fine. BUT... every time Plesk 11 rebuilds main.cf, this setting will be overwritten to:
virtual_transport = plesk_virtual

One can trigger the rebuild by calling this from bash:
/usr/local/psa/admin/sbin/mchk --without-spam


I have not found anything in the psa-database about virtual_transport or plesk_virtual.
How to achieve that the setting virtual_transport = relay will remain as desired and unchanged in the future?