The Return address, also called « RFC5321.MAILFROM » or « Envelope MAIL FROM: », is one of the few information that must be provided for an email to be sent. The related SMTP command is « MAIL FROM: », not to be confused with the « From: » header.
In the source of received emails, the header « Return-Path: » will contain the email address provided in the SMTP command « MAIL FROM: ». For this reason, the return address can also be called « Return-Path ».
This address is initially used as a return address, which will in some cases be sent a non-delivery report (NDR, for "Non-Delivery Report"), but is also used in the context of SPF authentication .
Unless otherwise, the same domain will be used for the visible address of the sender and for the return address.
In the analogy of paper mail, this address would be the return address applied in the back of the envelope.
This address is configured during the initial setting of your universe, after setting up the DNS entries. It is possible to change it upon request.