I ran into an unusual problem. I had to change all the IP's at work when I installed openvpn-as so that the IP's wouldn't conflict with home users. After the server rebooted, it no longer accepted smtp connections from outside itself.
Here's some of the checking I've done:
netstat -nl | grep 25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
iptables -L | grep smtp
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:smtp
The server is on a private IP with the router forwarding all traffic from one of our public IP's to the private IP.
I can telnet into port 25 using the public IP from the server, and I get:
220 mydomain.com ESMTP Sendmail 8.14.3/8.13.8; Tue, 26 Jan 2010 16:07:20 -0800
*** Domain name changed to protect the innocent...
Yet if I try to do the same from outside, I can't connect. I am still receiving e-mail. But I can't send mail from any computer except the server. I used to be able to send mail from anywhere.