I couldn't answer this myself, so here's an answer from this website: http://www.englishforums.com/English/SimplePastPastPerfectTense/dnjzb/post.htm
When telling the story of something that happened in the past, you may want to refer back to situations which were in effect before the time the story happened. In that case you use the past perfect. Without establishing the past point of view there's no need for the past perfect tense.
You would have to use the past perfect in reporting the present perfect, thus:
Direct quote: "I have sent an e-mail".
As reported later: I said that I had sent an e-mail.
-- I'm waiting for a response.
-- A response?
-- Yes. I sent an e-mail yesterday, and now I'm waiting for a response.
As reported later: I was waiting for a response to the e-mail I had sent the day before.
(I was waiting establishes the past point of view. had sent is the past of that past, so to speak. It is even 'more past' than the past.)
A couple other websites for you to look at: