By default Sendmail ships with a setting that lets it accept localhost/loopback (127.0.0.1) connections only. To make sure that you can connect to it on the external ethernet interface do the following:
- Open /etc/mail/sendmail.mc in VI or any other text-editor of your choice
- Find a line that looks like this "DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl" (or for other/older releases "O DaemonPortOptions=Port=smtp,Addr=127.0.0.1, Name=MTA")
- Replace the IP address after "Addr=" with the IP address of your external adapter, or remove "Addr=127.0.0.1," altogether to allow connections from all interfaces. You can also change the value of the "Port=" (default "smtp" = "25").
- Save the file (if you're using VI, Esc+:wq)
- Execute the following command to recompile Sendmail with new configuration: "make -C /etc/mail"
- Restart Sendmail using "service sendmail restart"