Assume the following scenario:
Your Postfix MTA is located in the Internet (having a public IP address) and you have configured an external DNS server there to be used for DNS requests. But you are going to send mails using an internal mail server as mail relay. In this case the private IP address of the internal mail relay cannot be re-solved by the external DNS server. In this situation Postfix cannot use the mail relay and shows an error message like this below in the log file:
Aug 26 12:49:12 myclient postfix/error: B35AF34B: to=<firstname.lastname@example.org>, relay=none, delay=0.14, delays=0.09/0/0/0.04, dsn=4.3.5, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=mailrelay.company.local type=AAAA: Host not found)
To re-solve the issue you have to add the following statement to main.cf
disable_dns_lookups = yes
and re-load postfix:
service postfix reload
Use the appropriate command to enforce delivery of queued mails.