Secunia Security Advisory – plonk has discovered a weakness in Mozilla and Thunderbird, which can be exploited by malicious people to enumerate valid email addresses.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/34944/sa13086.txt
Source: https://packetstormsecurity.com/files/34944/Secunia-Security-Advisory-13086.html