A critical RCE vulnerability in the Exim mail transfer agent (MTA), tracked as CVE-2018-6789, affects most of the email servers online. It has been estimated that as in March 2017, the total number of Internets email servers running Exim was over 560,000, that corresponds to 56% of all Mail (MX) Server online. According to Shodan, the number of Exim Servers exposed online is more than 4 million, most of them in the US.”]
Source: http://securityaffairs.co/wordpress/69954/hacking/exim-mta-flaw.html