The Sender Rewriting Scheme preserves .forward functionality in an
SPF-compliant world.

SPF requires the SMTP client IP to match the envelope sender
(return-path). When a message is forwarded through an intermediate
server, that intermediate server may need to rewrite the return-path to
remain SPF compliant. If the message bounces, that intermediate server
needs to validate the bounce and forward the bounce to the original
sender.

SRS provides a convention for return-path rewriting which allows
multiple forwarding servers to compact the return-path. SRS also
provides an authentication mechanism to ensure that purported bounces
are not arbitrarily forwarded.

Homepage:
https://www.libsrs2.org/
