From https://en.wikipedia.org/wiki/Exim:

Exim is a mail transfer agent (MTA) used on Unix-like operating systems. [...] In August 2019 [...] approximately 57% of the publicly reachable mail-servers on the Internet ran Exim.

Yikes.

edit: This might be a better summary of the vulnerability: https://www.tenable.com/blog/cve-2019-15846-unauthenticated-...