The remote mail server is affected by multiple denial of service vulnerabilities.
The remote host appears to be running Ability SMTP Server. According to its banner, the installed version of Ability Mail Server is affected by two issues that could cause the application to crash. One involves messages that are changed to a blank string, the other concerns IMAP4 commands with malformed number list ranges. It is not currently known whether either or both issues can be exploited without authentication.
Upgrade to version 2.61 or higher.