I have been receiving a fair amount of Spam from an e-mail forwarder. They are unwilling to correct their problems. Much of this Spam is in the form of bounce notifications. Attempting to reject other Spam resulted in more notifications. To control this Spam I implemented signed return path addresses. As a side benefit, I am also rejecting bogus notifications sent directly to me.
Signing my return path allows me to reject faked notification e-mail. The SMTP standard requires that no email sent with a null return path “<>” (aka Envelope Sender) be returned. Its purpose is for allowing for notifications about existing messages. These include notifications such as address unknown, message delivered, and message read. E-mail notifications which are not about a previously sent message can be refused. Signing the return path allowed me to reject such invalid notifications.