The problem can be corrected by updating your system to the following package versions: Ubuntu 16.04 LTS: sa-exim 4.2.1-14+deb8u1build0.16.04.1 In general, a standard system update will make all the necessary changes.
https://ubuntu.com/security/notices/USN-4520-1
CVE-2019-19920
https://launchpad.net/ubuntu/+source/sa-exim/4.2.1-14+deb8u1build0.16.04.1
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