It seems you should still be able to track down the offender. I don't think RCU allows you to see other member's email addresses, so the offender must have used the form for sending email. Just look at the logs of who sent email via RCU to the member.

Personally, I see no need for the email option at all. Private messages work fine, and they kind of "stay in the forum". They allow one-to-one messages, but are not really "back door".