Microsoft Outlook Express before 4.72.3612.1700 allows a malicious user to send a message that contains a .., which can inadvertently cause Outlook to re-enter POP3 command mode and cause the POP3 session to hang.
https://euvd.enisa.europa.eu/vulnerability/EUVD-1999-1014
http://www.securityfocus.com/bid/252