interface to Sender Rewriting Scheme

 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.