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.
To set the port, append
:, 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.
