The sid-milter package is an open source implementation of the Sender-ID and SPF sender authentication systems. Sender-ID and SPF have both been published by the IETF (RFC4406 and RFC4408, respectively) as experimental proposals. This package consists of a library that implements the record parsers needed to provide the authentication service, and a milter-based filter application that can plug in to the sendmail MTA to provide that service to sufficiently recent sendmail and postfix MTAs. An optional asynchronous resolver library is also provided to work around limitations of the basic BIND resolver which comes installed on most systems.