Add this lines to /etc/postfix/transport
domain.com :
.domain.com :
* smtp:[host.domain.com]
line1: domain.com should refer to your domain.
line2: dont forget the dot at the begining.
line3: the host is the name of your linux-box host name.
after adding this lines execute this command:
postmap /etc/postfix/transport
(this adds the lines added in /etc/postfix/transport to transport.db)
then restart the postfix
/etc/init.d/postfix reload
(last command may be different in various linux distros)
useful logs may be founded in /var/log/mail
mail.err
mail.warn
and very good documentation on www.postfix.com