Answer
Normally GMS will read the DNS information you have already set up in your Operating System and use this, so you should only ever need to set this if you would like GMS to use a different set of DNS servers from that already defined.
Go to the Outgoing > MX tab of the Administration Interface. Add an entry for the DNS server you would like to use in the appropriate text area and click on Update.
You need to ensure that the DNS servers specified are always available. A lack of access to DNS will cause all of your outgoing mail to be delayed (or fail altogether after a period of time). It may also interfere with incoming mail depending on which settings you have configured.
Note that the DNS server address must be specified in dotted decimal format (by IP address). Multiple DNS servers may be specified by separating each entry with a space.
Providing more than one DNS server in this list (or directly in the OS) will cause GMS to use the servers in a round-robin fashion for requests, it will therefore send more requests to the DNS server that responds fastest (so if one machine is more heavily loaded by other DNS requests, GMS will automatically balance its requests).
To further enhance DNS lookup performance GMS automatically caches the most recent requests which further helps to reduce the load on your DNS servers.
Keywords:DNS server not defined Post Outgoing Block Send queue