4.3.1. RETURN-PATH

This field is added by the final transport system that delivers the message to its recipient. The field is intended to contain definitive information about the address and route back to the message's originator.

Note: The "Reply-To" field is added by the originator and serves to direct replies, whereas the "Return-Path" field is used to identify a path back to the originator.

While the syntax indicates that a route specification is optional, every attempt should be made to provide that information in this field.