Recently I noticed some emails sent by Magento keep bouncing back. The mail server dropped me a few "Undelivered Mail Returned to Sender" messages. This is very undesirable, since this means that some order confirmations and/or invoices are not being delivered to the customer. After some investigation I discovered a Magento setting under System > Configuration > Advanced > System > Mail Sending Settings > "Set Return-Path". Set this value to "Yes" to make the Return-Path [...]