DNS forwarder issue

Hi friends,
I have windows exhmail server also i have isa,dns and symantec brighmail smtp....in my smtp server i saw name forwarder option internal dns or external dns ,,,which one i need to put there for better performance ? internal is with a dns forwarder configured...or i should give external dns server ip for fast mail delivery in that smtp server...advice me please.

I am also facing email delivery delay and failure problems. Can anyone help me on this issue?