When using Jenkins internally, it is sometimes needed to restrict sending e-mail notification external.
There should be possible to add allowed domains in which to send e-mails to.
When using Git as VCS the e-mail address of the committers is stored with the commit, and the mailer will the try to send to all users that might have broken the build.
If you have cloned an open source project and regularly pulls updates from upstream the mailer might start sending mails to those upstream committers.
That is not what you want from an internal Jenkins instance.