Speak with Andron Gant in the Undercity.
The letter of introduction should be enough to convince Andron that you are the messenger he's been waiting for. I've met Andron before, he doesn't quite have the mental capacity to be that suspicious.
Lord Varimathras suspects that Andron has been providing aid to some other group. He wishes to know what organization this is and requires proof of it.
I want you to speak with Andron, and obtain this information. You'll find him at his shop near the Apothecarium.