The remote mail server is prone to a script injection vulnerability.
The remote host is running Alt-N MDaemon, a mail server for Windows. According to its banner, a version of MDaemon mail server older than 10.0.2 is installed on the remote host. Such versions ship with a version of WorldClient (a webmail client) that is affected by a script injection vulnerability. By tricking a user into opening a specially crafted email, an attacker can exploit this issue to execute script code in the user's browser in the security context of the affected application and thereby steal cookie based credentials or launch other attacks.
Upgrade to version 10.0.2 or higher.