source: http://www.securityfocus.com/bid/252/info

A vulnerability in Outlook Express allows a malicious message sent to the users mailbox to halt POP mail download. The Microsoft tracking number of this issue is S2134 T6142.

A line with a single dot ('.') stands for EOM (end of message) in the POP3 mail transport protocol. If a message contains a single dot in a line is must be escaped by added an extra dot. However if at the IP level a line containing two such dots falls at a packet boundary such as the one dot is at the end of one packet and the other dot is at the end of the next packet Outlook Express will interpret the dot in the second packet as the EOM marker. This result in Outlook Express switching back to POP3 command mode and interpreting the rest of the message as a POP3 response. This normally result in an error message or the hanging of the session. The message will not be removed from the server and any further attempts will meet with the same result. 

Include a few thousand lines like this in an email and the bug will trigger:

So
.
this
.
might
.
actually
.
cause
.
the
.