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+libsrs2 is the next generation SRS library from the original designer
+of SRS.
+
+It can be used by Courier IMAP.
+
+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.
+
+
+What is SPF?
+
+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.net/.
+
+[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.
+
+What is SRS?
+
+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.