A smart host is simply the SMTP server of another party, often your ISP, that is preferable to your own for reasons such as reliability or credibility.

Mac OS X Server 10.6 allows you to configure such a smart host in Server Admin.

To set the port, append :<port>, for example smtp.example.com:587.

You may need to authenticate to the smart host. This is also possible. However, your email may still not go through—and often it is because the smart host expects a STARTTLS command. This is true for GMail SMTP.

Check if you find 530 5.7.0 Must issue a STARTTLS command first. in /var/log/mail.log.

If this is the case, try to append these two lines to /etc/postfix/main.cf:

smtp_tls_security_level = may
smtp_sasl_security_options = noanonymous

Then issue postfix reload.