Check /var/log/mail.log You might want to review Postfix install instructions for your version of Linux. The Linode help docs are usually really good: http://library.linode.com/email/postfix/gateway-ubuntu-10.10-maverick
But I think this Ubuntu one might be more helpful: https://help.ubuntu.com/community/Postfix The server name is going to be your server's name. As suggested, try running: sudo dpkg-reconfigure postfix
