Zimbra: changing postfix parameters

I found the solution to my own problem. By looking at the file /opt/zimbra/conf/zmmta.cf, I saw the following lines:

if VAR zimbraMtaMyNetworks
    POSTCONF mynetworks VAR zimbraMtaMyNetworks
fi

With zmprov I can check the current settings and change them:

zmprov getServer servername.mydomain | grep zimbraMtaMyNetworks
zmprov modifyServer servername.mydomain zimbraMtaMyNetworks "127.0.0.0/8 [...]"
postfix reload

And the problem is solved. Xeon suggested an alternative and simpler way:

You can also change this in the Admin Console under Server Settings -> MTA -> MTA Trusted Networks