I just setup
postfix but I just realized my ISP outgoing blocks port 25 (SMTP). How do I send email outside? I can see my mail queue is filling up. Any suggestion?
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.
If you don't have a static IP address, you shouldn't be sending email directly to the Internet. Kudo's to your ISP for blocking the spambots on their network from reaching me. Even if you were allowed to send email, many hosts would refuse your mail or route it to the spam box. Configure you mail server to send via your ISP's relay server. Many configurations have an option for using a smart host for delivery via a relay.
If you get a static IP address, it is likely your ISP will allow you to send to port 25. However, if you want your mail delivered there are several things you should do. You may want to review my Email Policy or my rant on Running an Email Server.