Some companies and ISP’s do not allow you to send mail from your machine directly, but force you to use their smarthost. In the Postfix SMTP server we can accomplish this by editing the file /etc/postfix/main.cf:
relayhost = smarthost.example.org
We also have to tell Postfix to which domains it may send mail. We do this by adding:
relay_domains = example.org
These two lines together allow your Postfix installation to send mail to email@example.com through the smarthost smarthost.example.org.
Configuring Postfix to use a smarthost