If your ISP is blocking outbound port 25, you've probably made a bad choice in the location of your mailserver. Because port 25 is blocked, I am assuming you have a machine inside your home/office, vs using a server.
I would _highly_ recommend that you take what you've learned from setting up postfix and install it on a cheap VPS. It is not a good idea to have an intermittent mailserver, or one that jumps IPs.
Otherwise, your option is to get a cheap VPS and create a tunnel, but in doing this you'll most likely have to add the tunnel's ip to a TXT spf record, which starts complicating things.