The remote host is vulnerable to a buffer overflow.
The remote host is running Alt-N MDaemon, a SMTP/IMAP server for Windows operating systems. It is reported that versions up to and including 6.5.1 are prone to multiple buffer overflows. An attacker may deny service to legitimate users or execute arbitrary code on the remote server. The attacker needs to authenticate in order to exploit these vulnerabilities against the IMAP server but it doesn't need to do so against the SMTP server.
Upgrade or patch according to vendor recommendations.