When Sending an Email from a Task Object the user receives a "Batch Failed Error" message.
This is because the Task object has many types and can also be linked to Users or Contacts and the fields are linked dynamically by Salesforce.
Salesforce API documentation says "This field contains a value only if the related record is a user." Sending an Email from a Task Object cannot be supported due to Salesforce API limitations. See API Developer Guide for more information.