Kolab / Postfix : relay acces denied

[Damien Ulrich]

Hi all,
I've got a functionnal server with Postfix/Cyrus/sasldb/..., 
its own DNS server with MX entry pointing on itself,
with which I can send/retrieve mails on the net, as locally
(on debian sarge)

I've installed and configured kolab(2)/postfix/ldap/...,
on a another computer,
working well with kontact/kmail/korganizer/...
which can only send mails locally
(on debian etch -with corresponding packages-)
kolab version : 2.2.12

I'd like now to give the second one the same functionnality as the first
I mean sending mail to external adresses
(to save energy)
But always have "relay access denied".

I tried :
postconf > file, on both configurations
then 'diiff' on the files to compare them
But can't find what's (really) missing for now

I had to search on the kolab-webadmin pages...

Is it possible to do so without breaking groupware functionnality ?
Is the solution in postfix configuration ?

Thanks per advance !
Dams

PS : I hadn't posted details because I'm not sure what's necessary, and it 
would be too long...
Feel free to ask for more...