Is port 25 unblocked on your firewall to the outside world? You might want to use
nmap or similar from an external pc to see if port 25 is open.
Even so, most ISPs disallow running a mail server via their networks. And you open yourself up for attack.
I use
fetchmail, which I installed from the Fedora repository. I run it as a cron job. It goes out to all my external email addresses every few minutes and fetches the waiting email and delivers it to my PC's port 25. Port 25 is blocked by my router but fetchmail is running locally, so it uses the localhost interface.