Hello Friends, I am using Ubuntu VPS with ISPConfig3. I am using firewall UFW. After enabling the firewall I am not able to send an email on outer domains but it's working with the same machine. Rest every thing is working fine. Even When I am disabling to firewall email is delivering to other also. My resolv.conf etc are configured correctly. Host or domain name not found. Name service error for name=gmail.com type=MX: Host not found, try again Ports are opened both tcp/udp protocols.