The remote host is vulnerable to a buffer overflow.
In version 4, a buffer overflow was introduced into the qpopper source tree. This buffer overflow is related to the handling of the client-supplied username and is present when a POP3 session is being initiated. It is believed that the overflow occurs before authentication, so it may not be required that users have valid POP accounts. This vulnerability can lead to a compromise of root privileges to remote attackers.
Upgrade to the latest version.