SharePoint 2013 designer workflow error 400 or Email activity error

RequestorId: 6b3b2ca8-21cd-122b-0000-000000000000. Details: An unhandled exception occurred during the execution of the workflow instance. Exception details: System.ApplicationException: HTTP 400 {"Transfer-Encoding":["chunked"],"X-SharePointHealthScore":["0"],"SPClientServiceRequestDuration":["79"],"SPRequestGuid":["6b3b2ca8-21cd-122b-8415-1dbd28108890"],"request-id":["6b3b2ca8-21cd-122b-8415-1dbd28108890"],"X-FRAME-OPTIONS":["SAMEORIGIN"],"MicrosoftSharePointTeamServices":["15.0.0.4551"],"X-Content-Type-Options":["nosniff"],"X-MS-InvokeApp":["1; RequireReadOnly"],"Cache-Control":["max-age=0, private"],"Date":["Thu, 27 Feb 2014 08:20:02 GMT"],"Server":["Microsoft-IIS\/8.0"],"X-AspNet-Version"

This error mainly comes when the email ID is not registered to any of the User in SharePoint. You can definitely send email to any of the Email ID of the world. But that Email ID need to be registered in any of the Email ID of the User.
Email ID of a user can easily be changes by using the power shell  to update the user detail information in User Information list of the site. Or it can be changed by Updating the User profile information from the Central Admin.
Power shell command to update the User Email:
Set-SPUser -Identity 'yourmachine\Roberto' -DisplayName 'Roberto' -Email 'robertosljunior@yahoo.com.br' -Web http://yourmachine