Satya's blog - Tell postfix to send by port 587

The Postfix SASL documentation says:

The "submission" destination port tells Postfix to send mail via TCP network port 587, which is normally reserved for email clients. The default is to send mail to the "smtp" destination port (TCP port 25), which is used for receiving mail across the internet.

So when my Postfix was trying to send to my (non-ISP) smart host on port 25, and my DSL provider was blocking 25, I had to add the port in main.cf:

relayhost = relayhost.example.com:587
smtp_sasl_password_maps = hash:/etc/postfix/sasl_map

And in my sasl password map file, instead of just relayhost.example.com, I needed to add the ":587" part, and then regenerate the db file:

postconf /etc/postfix/sasl_map