This facility is listed as OpenBSD-specific as it depends on both the pf(8) facility and the smtpd(8) daemon’s helper program, smtpctl(8). That said, the output of smtpctl spf walk is text, and text is easy to reform. If you find a way to use this on other platforms, great - we’d love to be cross-platform. When the internet was young, everybody ran their own mail server - singular. SPAM came along. GreyListing was one of several good responses to it. Then big mailers grew multiple servers AND were too lazy to implement "server affinity" - keep trying to send a given message from a given IP. Instead, they send a message from a randomly-chosen server each time they retry after the greylisting delays it… This broke greylisting, as the message would come from a different IP each time, which would get added to the ever-growing grey list, but the message would rarely get through.