[otrs] Auto-inserting user's email address in response

Hi, Running OTRS 2.0.4 on RedHat Enterprise. I have created a Response and I would like to insert the customer's email address back into the body of the message. I am looking for the proper substitution key. The bottom of the "Add Response" form in the Admin section hints that I can use "" but this results in nothing (just a "-" in the message). I grepped through the source code and I found "OTRS_CUSTOMER_EMAIL" but it looks like this is the actual body of the message, not the user's email address. So, which variable represents the user's email address? I see _REALNAME, but no _EMAIL_ADDRESS or something... Thanks! /Cody