Reference Implementation: Mail::SPF::Query
The Perl module Mail::SPF::Query has been the very first implementation of SPF and is currently the de facto reference implementation. It roughly conforms to the final SPFv1 specification, however due to its long heritage (reaching back to 2003) there are some deviations. The Mail::SPF::Query package includes a comprehensive test suite, a command-line query tool, a query server, and sample plugins for several mail servers. The library also supports a number of non-standard features, such as best-guess processing and trusted forwarder accreditation checking.
The project is currently working on a new Perl module, Mail::SPF, to replace Mail::SPF::Query as the reference implementation (ETA June 2006).
Libraries
Extensions for Mail Clients
Extensions and Patches for Mail Servers
Other Software that supports SPF
Please report SPF-supporting software to the spf-discuss mailing list.