You must put in the DNS gateway IP addresses under Setup => Network. If you do this, and have the correct SMTP server it should resolve your problem.
If you don't know what DNS gateway to use, the DLINK customer support told me to put 4.2.2.2 and 4.2.2.3 in the primary and secondary gateways. once I did this, I was able to get the email snapshots and alerts to work when I did the "TEST" function.