Qpopper 2.53 and earlier allows local users to gain privileges via a formatting string in the From: header, which is processed by the euidl command.
http://www.securityfocus.com/bid/1242
http://www.novell.com/linux/security/advisories/suse_security_announce_51.html
http://archives.neohapsis.com/archives/bugtraq/2000-05/0267.html