sendmail Doesn't Listen

sendmail service on CentOS default installation listens only on the loopback interface, for obvious security reasons. To modify this to listen on all binding ip address change the line

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

to

dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

As usual with any configuration change, recompile and restart service

> m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
> /sbin/service sendmail restart

Ensure sendmail.cf has rw-r--r-- permissions only, or it will complain about "dangerous write permissions".