Configure Postfix (SMTP Server) on my Server

Hi there, I would have a small job to give. I am having a server, for OS debian linux stable, which is administered by webmin. Now I tried to setup postfix on that server through webmin, but somehow I obviosly didn´t do the right configuration, so that postfix doesn´t work properly, meaning all emails sent are not delivered for "unknown mail transport error" according to mailqueue. Now I would need someone to fix the settings there, so that postfix would work. I guess for someone who knows what he does that is a job of probably half an hour, probably one hour, hence my budget on that one is rather limited. Thx for your bids.