A simple plugin that, while activated, logs all calls to the wpmail() function to a file wp-content/wp\mail_log.
Please be advised, this file is probably publicly accessible on your server, and could well contain private information. This plugin is for debugging purposes only, I encourage you to delete the wp_mail_log file and deactivate the plugin once you’re finished with it.
If you have a support question, try posting a comment below.
Get the code here.