We have a postfix server configured in a way that it is not allow to send/receive emails from outside our company and it is not connected to the internet. When users send/receive emails outside our own domain, they will receive the warning email "#4.0.0 X-Postfix; Host or domain name not found. Name service error for name=xxxx.com type=MX: Host not found, try again>". The problem is that they can only receive the warning email 1-2 hours after they send the email. It seems that the server is trying to lookup the outside world for dns records. What can I do to shorten the time users receive the warning email. thanks

I sent an email to a non-existing domain @yyy.com for testing and the line "Jan 23 10:11:44 mail postfix/smtp[32423]: 150422D606: to=<alex@yyy.com>, relay=none, delay=20, status=deferred (Host or domain name not found. Name service error for name=yyy.com type=MX: Host not found, try again)" appeared in /var/log/mail almost instantly, but the email client just didn't receive the warning email until 1-2 hours later.