The remote Debian host is missing a security-related update.
Meh Chang discovered a buffer overflow flaw in a utility function used in the SMTP listener of Exim, a mail transport agent. A remote attacker can take advantage of this flaw to cause a denial of service, or potentially the execution of arbitrary code via a specially crafted message.
Upgrade the exim4 packages. For the oldstable distribution (jessie), this problem has been fixed in version 4.84.2-2+deb8u5. For the stable distribution (stretch), this problem has been fixed in version 4.89-2+deb9u3.