The remote host has an email client installed that is vulnerable to multiple attack vectors.
Versions of Mozilla Thunderbird prior to 18.104.22.168 are affected by an SSL spoofing attack. The client can be fooled into trusting a malicious SSL server certificate with a null character in the host name. (MFSA 2009-42)
Upgrade to Thunderbird 22.214.171.124 or later.