All the content of other websites but none of the music!
SPF is a protocol to try to provide some defense against domain spoofing. You can turn on SPF checks in courier-mta by adding various things to bofh. You might want to begin by reading the SPF RFC and the Courier SPF docs. Possible Keywords (descriptions from the RFC): Keyword Description “none” A result of None means that no records were published by the domain.