When the extended e-mail plugin is configuered to "Include Culprits" and "Send to Committers", then an e-mail is generated, which includes everyone, who committed since the last successful build.
During the development of the software we have an unstable build for a longer period. During this time, someone of our team has left the company and now the plugin tries to send an e-mail to this non-existent person, causing an error in our e-mail system. Also, this person is generated in the "People" section of Jenkins.
I would suggest to check, if these persons have a valid account in the "People"-database and to avoid to add these people to the "People"-database and to avoid sending e-mails to these persons.