The remote Debian host is missing a security-related update.
The Qualys Research Labs discovered a memory leak in the Exim mail transport agent. This is not a security vulnerability in Exim by itself, but can be used to exploit a vulnerability in stack handling. For the full details, please refer to their advisory published at: https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt
Upgrade the exim4 packages. For the oldstable distribution (jessie), this problem has been fixed in version 4.84.2-2+deb8u4. For the stable distribution (stretch), this problem has been fixed in version 4.89-2+deb9u1.