I am not sure if I am having a firewall (iptables) issue or a Postfix issue.
When I try to send email to my linode, it get "No connection could be made because the target machine actively refused it __IP__:25"

If I use telnet on the server itself, everything works fine.

The reason I do not think that it is iptables' fault is because I ran a port scan:
Ports blocked by iptables drop traffic, while this one, which is not blocked actually rejects traffic.

(iptables rule: ACCEPT tcp -- anywhere anywhere multiport dports smtp )

I do not have a lot of experience with postfix, but I was following a guide on the linode library to set it up.

System: Ubuntu 10.04 LTS i686

Many ISPs block outgoing port 25 to prevent abuse - it is usually best to set up postfix to use smtps and/or submission ports for originating mail from external clients.

I am trying to set up a real mail server (so I can pipe mail to a php script, if it matters).
And, I can connect to other servers on port 25 with no trouble.

Is a bounce message better?

OK, I misunderstood the original post.

Is postfix running?

# netstat -ltpn

Should have a line like this:

tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      6705/master

It appears to be running, but not bound to the main IP.
Since I am not familiar with Postfix (my usual mail program is Microsoft SMTP/Exchange), I planned on using Webmin to manage postfix.
How do I change the listening port?

It looks like you are only binding to localhost. You can set which interface postfix will bind to with "inet_interfaces" in main.cf. Normally you would use

inet_interfaces = all

Not sure what you would do in webmin...

Does it only bind to interfaces or can I use IP bindings like Apache?

Thanks so much.

Thank you.