If you are still using sendmail software in system and getting an error below, 

# echo "Test Email from sendmail server" | mutt -s "Test email from server"  linuxfaq@emailids.com


Error sending message, child exited 69 (Service unavailable.).

Open your host allow file, add the below line,
 
# vim /etc/hosts.allow


ALL: 192.168.0.0/255.255.255.0

sendmail:127.0.0.1,192.168.0.1

or
sendmail:ALL


Note : I am not sure about it, if this is the most secure will you add second  configure (sendmail:ALL).