The remote host is missing a critical security patch or upgrade.
The remote host is running a vulnerable version of Mozilla Thunderbird mail client. This version of Thunderbird is vulnerable to multiple flaws that might allow an attacker to launch Denial of Service attacks, buffer overflows, information disclosure, privilege escalation, and other issues. An attacker exploiting these flaws would need to be able to convince a user to open or read a malicious email. Successful exploitation would, at the most, give the attacker the ability to execute arbitrary code with the permissions of the user running Thunderbird.
Upgrade to version 184.108.40.206 or higher.