The remote host is running a vulnerable email client.
The remote client is running Kmail, an email client for Unix and Unix-like operating systems. This version is vulnerable to a content-parsing flaw within the HTML handlers. Specifically, a client with HTML enabled may be sent a malicious email that is able to overlap portions of the displayed email. This can be used to convince users to perform web-based tasks that have unexpected results.
Upgrade or patch according to vendor recommendations.