You shoud have a Moodle with two students and both students have email notifications enabled for messages (you can access as admin to "Site administration > Messaging > Notification settings" and then enable Online and Offline "Personal messages between users "in the Email column). Open mailhog, mailcatcher or similar tool configured to capture emails from your Moodle site (or set one up). Note: Don't test it using Gmail. It works fine with and without the patch. Log in as student 1. Send student 2 this message, eg "Hello, السلام عليكم". Open mailhog and CONFIRM that you see the same message, including "This is a copy of a message ..." message at the end. Send student 2 another message, this time with some HTML and ending with an opening HTML comment, eg: <h1>Important message from Moodle</h1> <img src="https://moodle.org/theme/image.php/moodleorg/theme_moodleorg/1621925120/moodle_logo_small"><!-- Check Mailhog and CONFIRM the "This is a copy of a message..." text is visible.