your netstat -tulp result:
tcp 0 0 *:smtp *:* LISTEN 17628/master
clearly shows that the server is listening to all interfaces for SMTP, that's what *:smtp means.
The first telnet you tried with the -l flag is a syntax that seems odd to me. I would try just "telnet localhost 25" and also a "telnet 127.0.0.1 25" and then copy and paste the results. It would be useful to see if you get a connection refused or a timeout.
Also, many versions of linux now come with forms of security, like selinux on red hat and others have firewalls. So even if the OS thinks it's listening you may have to configure the firewall to let IP traffic get to that listening port.