POP3 Server for Ipswitch IMail 7.04 and earlier generates different responses to valid and invalid user names, which allows remote attackers to determine users on the system.
http://www.securityfocus.com/bid/3424
http://www.ipswitch.com/Support/IMail/news.html
http://archives.neohapsis.com/archives/bugtraq/2001-10/0076.html