The remote host is missing an update to dovecot announced via advisory DSA 1359-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201359-1

It was discovered that dovecot, a secure mail server that supports mbox and maildir mailboxes, when configured to use non-system-user spools and compressed folders, may allow directory traversal in mailbox names. For the stable distribution (etch), this problem has been fixed in version 1.0.rc15-2etch1. For the old stable distribution (sarge), this problem was not present. For the unstable distribution this problem with be fixed soon. We recommend that you upgrade your dovecot package.