The remote host is vulnerable to a buffer overflow.
The remote host is running Alt-N MDaemon, an SMTP/IMAP server for Windows. According to its banner, the version of MDaemon on the remote host is prone to a remote buffer overflow. Versions of Alt-N prior to 8.1.4 are vulnerable. The attacker exploiting this flaw would send a malformed request before the authentication process. Successful exploitation would allow the attacker to execute arbitrary code.
Upgrade to version 8.1.4 or higher.