So, what's the mail flow? Email system -> NDLP -> ESA?
Assuming that's the case, on the ESA, click on Mail Policies/HAT Overview, at the top of the grid, select "OutboundMail" (The actual name could be anything, but the intent should be outbound). There should be a sendergroup there called RELAYLIST.
Add the IP of the DLP box to this, save/submit/commit. That will allow the DLP box to send mail through the ESA.
