This hook will allow you to filter the email headers that are sent through wp_mail.
(mixed)(required) The email headers.
(string)(optional) Toggle to determine what email is being generated (newreg|newmod|appmod|repass|admin).
Since version 2.7.4
Moved to /inc/email.php in 3.0.0
wpmem_email_headers is located in /inc/email.php