Jenkins Email Extension Plugin 2.75 and earlier does not perform hostname validation when connecting to the configured SMTP server.
https://www.jenkins.io/security/advisory/2020-09-16/#SECURITY-1851
https://www.jenkins.io/security/advisory/2020-09-16/#SECURITY-1851
http://www.openwall.com/lists/oss-security/2020/09/16/3
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-2253