libsrs2 is the next generation SRS library from the original designer of SRS. It implements the Sender Rewriting Scheme, a part of the SPF/SRS protocol pair. Libsrs2 has been written from an entirely clean codebase with compliance, speed and versatility in mind. It is platform independent and has no external dependencies. It is thread-safe and heap-safe, and is suitable for large scale applications and embedded systems and can operate without many standard system facilities.
SPF is one of a family of systems which aim to prevent email address forgery. A good SPF implementation is available from http://www.libspf2.org/.
[From spf.pobox.com]: The present SMTP standard for email allows anyone to forge anyone else's email address. This means I could send anyone a message claiming to be from you, and only an email expert would be able to tell the difference. SPF allows your mail servers to easily distinguish forgeries from real mail. Importantly, SPF works before the message body is transmitted, saving you the bandwidth cost of downloading the message and the CPU cost of filtering it.
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. A graphical explanation is available here or here and a technical document describing the protocol and its strengths is at here.
Contact the developers. Can you (for example)