Setup Log in as an admin. Create a new user s1. Visit 'Site administration' > 'Advanced features'. Turn on the setting 'messagingallusers'. Save the changes. Make sure that you can receive emails sent by your Moodle site. Recommended ways to do this: Configure Mailcatcher/Mailhog; OR You could add $CFG->divertallemailsto = 'youremail' in your config.php to catch all the emails sent by your Moodle site. Test 1: Send the first message to s1. A new conversation will be created. Log in as an admin. Search for a user s1 to send a message to. Send a message to s1 . Go to your inbox and check you have a new mail EXPECTED RESULT:  confirm that you don't have a duplicate end tag line as the error reported and only has at the end of the email this end tag line: .... --------------------------------------------------------------------- This is a copy of a message sent to you at "stable_master". Go to http://localhost/m/stable_master/message/index.php?id=IDUSERFROM to reply. Copy the URL which is in the en tag line. Log in as an s1. Paste the URL in the browser and press enter. EXPECTED RESULT: confirm that you can see the conversation between admin and user s1. Test 2: Send the second message to s1.  Conversation already exists. Repeat the same steps that test 1 and the EXPECTED RESULT has to be the same.