The remote Debian host is missing a security-related update.
Multiple security issues have been found in Thunderbird, which may lead to the execution of arbitrary code or denial of service.
Upgrade the icedove packages. For the oldstable distribution (jessie), these problems have been fixed in version 52.3.0-4~deb8u2. For the stable distribution (stretch), these problems have been fixed in version 52.3.0-4~deb9u1.