error in step "mail"

Hi Experts,

I am new to workflow.I created a simple workflow objet to send mail to specified user.To achive this i have create a new workflow object in that created a mail step in undefined step.In this step i have assiend the the user to recive the message.The recepient type as User and then i have gave the username. i haven't get any the syntax earrors.Then i have executed the workflow i got the error in Log like this: Unable to read RFC destination WORKFLOW_LOCAL_100.

please suuest to resolve this issue.