When you first install snf-milter, you would add the following line to your Postfix main.cf file:

smtpd_milters = unix:/snf-milter/socket

The above line will ensure the snf-milter runs on all ports.

However, if you would like it to run on only port 25 as some users do, you can do the following to accomplish that:

Removed this line from main.cf:

smtpd_milters = unix:/snf-milter/socket

Change/Add the following line for SMTP to your Postifx master.cf

smtp      inet  n       -       -       -       -       smtpd -o smtpd_milters=inet:127.0.0.1:12768 -o smtpd_milters=unix:/snf-milter/socket