The message_post_with_template() method is used to send mail with the Jinja template. If you just want to send an email with plain text, you can use the message_post() method:

self.message_post(body="Please return your book on time")

The preceding code will add the Please return your book on time message in the chatter. All of the followers will be notified with this message. If you just want to log the message, call the method with the subtype_id parameter.