SPF (and related systems) present a challenge to forwarders, since the
envelope sender address might be seen by the destination as a forgery by
the forwarding host. Forwarding services must rewrite the envelope
sender address, while encapsulating the original sender and preventing
relay attacks by spammers.

The Sender Rewriting Scheme, or SRS, provides a standard for this
rewriting which makes forwarding compatible with these address
verification schemes, preserves bounce functionality and is not
vulnerable to attacks by spammers.