Hey Falko, thanks for the reply! Unfortunately the problem is worse than this.
What I need is to set up my Postfix server to relay mails from a customer who shares an email domain between two servers, their company headquarters has an ISP in Finland where the domain is housed, and the branch office here in Sweden has an Exchange server they want to use with the same domain. They use POP3 to get their email and DNS to send, this works fine to everyone in the world except for their collegues on the same email domain. This is because Exchange thinks it is authorative for the entire domain and doesn't send emails out via DNS when it thinks it is local delivery.
The solution is using a smarthost, and this is where my Postfix server comes in. The ISP for the branch office here in Sweden does not provide a smarthost since there is no limitation on port 25 and they simply say "set up your own server". I thought of using the finnish SMTP-server but that doesn't work either, when you telnet to smtp.taajama.com, the server they use at the headquarters, you get a very strange response, it just doesn't seem to accept any commands sent to it... and it doesn't accept emails sent from the server here in Sweden.
In Sendmail and qMail which I've used in the past you simply add the domain you want to accept and relay for in a configuration file on the server and restart the mail daemon but things seem more complex in Postfix and I haven't been able to find a nice newbie guide to get it working.
Hope this clears up my question!