In some Exchange Server 2013 environments it will be necessary to manually configure the DNS servers that Exchange uses for external name resolution.
This is most common in environments where the internal DNS servers that the general server and workstation population uses for DNS cannot resolve external names (a security requirement for some organizations).
Obviously Exchange Server 2013 is a little different than your average server or workstation. DNS is a requirement for Exchange to be able to route outgoing email if a smart host is not being used.
To configure DNS lookups for an Exchange 2013 server in the Exchange Admin Center navigate to Servers, highlight the server you wish to configure, and click the Edit button.
In the DNS Lookups section drop down the list to either choose an adapter that has the correct DNS servers configured on it, or if no adapter has DNS servers configured that can resolve external names choose Custom Settings.
If you have chosen custom settings you can add the IP addresses of DNS servers.
Click Save to commit the changes.
The same configuration options also exist for internal DNS lookups.
Be cautious when configuring Exchange Server 2013 DNS lookups in this manner, and make sure you test the changes afterwards to ensure mail flow is still functioning correctly. Misconfigured DNS settings can cause problems with Exchange 2013 such as sent items being stuck in the OWA Drafts folder.