If you have an issue sending out an email from ZenCart such as order notification, regular marketing mails,… you may want to add this file into extra_datafiles, so that you’ll see the list of errors displayed on your browser.
If you’re sending email from ZenCart Admin, place the file inside admin directory.
– /myadmin/includes/extra_datafiles/email_debug.php
If you’re sending email from Customer interface, place the file inside includes/extra_datafiles directory. The file contains this line of codes:

 define('EMAIL_SYSTEM_DEBUG','5');// set to 0 to turn it off again