firewall works from the basis of any traffic its allowed, in other words all traffic its blocked unless you permit it with a rule.
you can forbid dns (udp 53) traffic to any server on the internet except opendns servers
your case its similar the diferences are:
change dns (udp 53) by smtp (tcp 25)
change open dns servers ip by your mail provider ip
if you dont know how to do that:

http://en.wikipedia.org/wiki/RTFM

dlink provides a lot of documentation right from the web gui using the help link on the upper rigth corner
port redirection its a nice way to explore and understand the way to do things with netdefend firewalls
nice reading