Prerequisites: Have a local mail setup installed, such as mailcatcher, to inspect email. Install a test tool that will verify the API extension:   git clone git @github .com:Peterburnett/MDL- 69724 -tester.git admin/tool/emailtester   Testing Instructions: 1) Set the default email settings for messages between users to online and offline here: /admin/tool/message.php 2) Create a test user to send messages to, which we will use as a test email. 3) Send a message to this user. 4) Verify that the email is received in mailcatcher. 5) Verify that the subject, from and contents all appear as they should. 6) Edit config.php and add a line to override the mail factory to one that inserts an extra header: $CFG->alternative_email_manager_class = '\tool_emailtester\email\manager' ; 7) Send another message to the test user. 8) Verify that the mail is received and everything looks the same as in Step 5. 9) Now inspect the source of the mail, and confirm there is an extra header in the mail: X-mail-testing: test. This confirms that the API extension is working.