Sorry, might have been confused as I forgot the un in unlikely. Other than that it should be quite clear really. You need to resolve anything that needs resolving, sorry if that sounds too obvious, but there's not much else to it. If you want to send email to firstname.lastname@example.org
then you'll need to resolve an mx record for yahoo.com and the A record for that mx. likewise if you want to send an email to dave@sales then the sales DNS records will need to be obtained from your internal DNS servers as it's clearly not resolvable externally. If you have no special interest in this, and only want to resolve global addresses, then you just need *ANY* dns server, opendns (188.8.131.52 and 184.108.40.206), google dns (220.127.116.11 and 18.104.22.168), your own isp DNS, whatever you like. You're only using it, not configuring anything about it.