The title of this question comes from the logs of SharePoint 2010. I defined a three-state workflow and the workflow gives errors when trying to send e-mail to the users participating in the workflow. I can't see anywhere 'firstname.lastname@example.org' is defined. The only configuration was Search Service app configuration and I changed the mail address to a valid one.
I had the same problem. In my scenario the error was caused because my sharepoint server was not authorized to send mails in my mail server.