You likely don't have issues with iptables, unless you installed and configured it as it is not running on a default Raspbian install. Also the default install will normally open and listen on the required ports. However, if you intend on running a mail server you should install and configure a firewall first, as your server will be a prime target for spammers etc. Properly securing a mail server involves considerably more than a firewall though.
As for port 25 It may be blocked by your ISP. I would suggest you implement SSL for your ingoing and outgoing mail which means that you will no longer be using port 25. The default configuration file should have automatically opened the port and be listening.
However, to answer your question you can see if the port is open and what process is listening with the following command:
sudo netstat -lptu
Additionally you can test your basic configuration using telnet as described in this post.
Depending on why you want an email server you may be better off just setting it up to forward all nonlocal mail through your Gmail account. I have not used it recently but this script makes that exceptionally easy.