input: 
  def send_email_task(recipient_name, recipient_email, subject, message):
      """
      Create a Celery task in Python to send an email with specified details.
      
      >>> from celery import Celery
      >>> from flask_mail import Message
      >>> app = Celery('tasks', backend='redis://localhost')
      >>> mail = connection_to_mail_service()  # Assume this function connects to the mail service
      >>> send_email_task('John Smith', 'john@example.com', 'Hello World', 'This is a test email.')
      'Task invoked: Email sent to John Smith at john@example.com with subject Hello World.'
      """
  
output:     @app.task
      def send_email(recipient_name, recipient_email, subject, message):
          msg = Message(subject, recipients=[recipient_email])
          msg.body = message
          mail.send(msg)
          return f'Task invoked: Email sent to {recipient_name} at {recipient_email} with subject {subject}.'
      
      return send_email.delay(recipient_name, recipient_email, subject, message)