This is driving me nuts. I have Postfix/Dovecot set up on Centos 6 VPS for my emails. I can send and receive emails without any problems on my outlook (home broadband), on my Android phone (3G connection), however when I set up exactly the same email account on Outlook at my friends house I'm getting bouncebacks when I'm trying to send email (receiving works fine):

554 5.7.1 : Relay access denied

Now if it wasn't working at all I'd understand, but why is it working on several internet connections, but not everywhere? Can someone shed some light on this? Where should I look for the problem?