不久前我也有同样的问题。

说实话，我还没完全弄明白。。。

出于所有目的，我找到了一个使用wp_mail, 然后从那里解决了这个问题：

wp-login.php: retrieve_password()
wp-admin/includes/class-wp-automatic-updater.php: WP_Automatic_Updater::send_email()
wp-admin/includes/class-wp-automatic-updater.php: WP_Automatic_Updater::send_debug_email()
wp-admin/includes/ms.php: update_option_new_admin_email()
wp-admin/includes/ms.php: send_confirmation_on_profile_email()
wp-admin/includes/upgrade.php: wp_new_blog_notification()
wp-includes/pluggable.php: wp_password_change_notification()
wp-includes/pluggable.php: wp_new_user_notification()
wp-includes/pluggable.php: wp_notify_postauthor()
wp-includes/pluggable.php: wp_notify_moderator()
wp-includes/user.php: wp_update_user()
wp-includes/ms-functions.php: wpmu_welcome_user_notification()
wp-includes/ms-functions.php: newblog_notify_siteadmin()
wp-includes/ms-functions.php: newuser_notify_siteadmin()
wp-includes/ms-functions.php: wpmu_welcome_notification()
wp-includes/ms-functions.php: wpmu_signup_blog_notification()
wp-includes/ms-functions.php: wpmu_signup_user_notification()