The remote host is vulnerable to multiple attack vectors.
The remote host is running a vulnerable version of Mozilla Thunderbird mail client. This version of Thunderbird is vulnerable to a Man-in-the-middle (MITM) or sniffing vulnerability where an attacker can determine user credentials even when encryption is enabled. To exploit this flaw, an attacker would need access to the local network. In addition, the client is vulnerable to a flaw where attackers can spoof attachment types. An attacker exploiting this flaw would craft a malicious email and entice a user to open a seemingly innocuous attachment.
Upgrade to version 1.5 or higher.