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.