My mistake, was a typo - should be /etc/postfix/main.cf
If comcast blocks port 25 then you are going to have trouble sending any mail via postfix. 25 is the smtp protocol port.
When Comcast tell you to use port 465 instead they are referring to when you use an email client, not a mailserver.
Your only hope is that Comcast have a mail server available that listens on port 25 so then you can use it as a relay (ie: your server becomes a "smarthost").
Sorry, I am not familiar with how Comcast runs their network, so I can't really help you much in this.