The Mailer and Email Extension Plugins are able to send emails to a dynamically created list of users based on the changelogs, like authors of SCM changes since the last successful build.
This could in some cases result in emails being sent to people who have no user account in Jenkins, and in rare cases even people who were not involved in whatever project was being built, due to some mapping based on the local-part of email addresses.
Affected versions: up to and including version 1.19
External Reference:
https://jenkins.io/security/advisory/2017-03-20/