SPF requires mail forwarding MTAs to rewrite the sender address. Shevek, the current maintainer of Mail::SRS, keeps a web page describing SRS in detail. You can download Mail::SRS from that page and from CPAN. His latest contribution (June 5th 2004) is libsrs2.

Mark has produced instructions for doing SRS in Sendmail; his instructions do SRS for all outbound mail in a VERP-like fashion. See also http://srs-socketmap.info.

James wrote the original libsrs. Miles Wilton has written an alternative implementation, libsrs_alt.

If you would like to discuss SRS, please join the SRS mailing list.

Click here for an article that tells email forwarding services (like acm.org or pobox.com) exactly what they need to do (or download a PDF version).

This article describes SRS in a little bit more detail.