I have never seen this mentioned in the forums and as far as I am concerned due to the extreme security problem that it can cause. LL should put it clearly in each and every instant message that is forwarded to an email account.
Here is the problem; Recieve an IM while offline and it is forwarded to your email. You recieve it and reply to it and it goes back through the SL servers to the correct account. If that person is online then no problem, they recieve the message. BUT if the person is offline and it is sent to thier email; THEY NOW HAVE YOUR RL EMAIL ADDRESS.
When you hit reply and it sends your response and the body of thier original message. This original message you recieved and are replying to has a hidden header showing the email address that Second Life forwarded the IM to. The only way to stop it is to make sure and not reply to the original message. Copy/Paste the reply to address into a new message and just the relevant part of thier original message to quote.
Brent Linden says that this is not an exploit. I do not agree with that accessment. Fortunately it is a freind that I now have the RL email address for. I do not think that you Torley, or Philip or anyone else would be too happy sending out your personal, home email address to the Second Life community. Evidently from the response I recieved, this was a known problem. Either put in a BOLD large type message in the footer of each IM forwarded to email, Disable IM to email(which won't happen), or put in a scrubber to delete the email addresses(RL not the key address) when they are forwarded to email.