According to its banner, the remote Cyrus IMAP server is vulnerable to a pre-login buffer overrun. An attacker without a valid login could exploit this, and would be able to execute arbitrary commands as the owner of the Cyrus process. This would allow full access to all users' mailboxes. More information : http://online.securityfocus.com/archive/1/301864
If possible, upgrade to an unaffected version. However, at the time of writing no official fix was available. There is a source patch against 2.1.10 in the Bugtraq report.