Every WordPress email usually uses
wp_mail(). Which is actually a wrapper of PHPMailer. Find out this
wp_mail() functions documentation and source code, you’ll find some hooks there which will be useful to you cases.
But keep it in mind that, those hooks will be applicable only if the mail is sent by using
wp_mail(). If any email is send by any other function like the PHP native
mail()function then the hooks may not work.