magento2: Return path e-mail variable system/smtp/return_path_email not used anywhere?

Trying to configure the return path e-mail as described on https://docs.nexcess.net/article/how-to-set-the-return-path-email-in-magento-2.html on Magento CE 2.1.0 but failing to do so.

I can see that both system/smtp/set_return_path and system/smtp/return_path_email are configured correctly in my core_config_data table but when I look for these variables in the source I don’t seem them used anywhere. The only references I could find for return_path_email are these:

vendor/magento/module-email/Model/Template.php:    const XML_PATH_SENDING_RETURN_PATH_EMAIL = 'system/smtp/return_path_email';
vendor/magento/module-backend/etc/adminhtml/system.xml:                <field id="return_path_email" translate="label" type="text" sortOrder="80" showInDefault="1" showInWebsite="0" showInStore="0">

Const XML_PATH_SENDING_RETURN_PATH_EMAIL doesn’t seem to be used anywhere either. Could someone please confirm that these settings are in fact used in Magento CE 2.1.0 & if so point me to the correct file for debugging?

Preconditions

1. Magento CE 2.1.0 on Debian jessie

Steps to reproduce

1. Configure return path as described on https://docs.nexcess.net/article/how-to-set-the-return-path-email-in-magento-2.html
2. Place order

Expected result

1. Return path being set correctly

Actual result

1. Return path remains www-data@domainname.com