Return-Path

The email address that should receive a MDN (message delivery notification) message if MDN is requested by the sender. This field is commonly added by the final transport system that delivers the message to its recipient. The field contains the email address of the original sender. The Reply-To field is added by the sender and is the email address for replies, whereas the Return-Path is where MDNs or bounced emails are sent. This field may be added by the sender to force bounced email and MDNs to be sent to an alternate address for automated bounce/MDN processing.

Example:

Return-Path: bounce-handler@chilkatsoft.com