Symptoms: A SharePoint Designer workflow that sends an email to a recipient fails with an error stating that the email address is not valid, although the name appears to resolve correctly within the People Picker field in an InfoPath form.
Cause: The user selected has more than one account with the same display name. In our case, it was a user with a normal AD account (with an email address) and an elevated system admin account (without an email address). Both accounts had the same display name.
Solution: The display name for the admin account was changed. After the next profile sync, the user was able to receive emails sent by the workflow.