SPF is a "solution" created by the big boys (AOL, verizon, and similar) whom handle tens of millions of mailboxes and even a 0.5% decrease in spam is a significant reduction in their costs (CPU, storage, heat, electric, A/C etc).
Yes, by design SPF and latterly DMARC, break the flexible nature of the 1970s SMTP standard, but its now ~40 years later, and the network is no longer a cooperation of universities but has all forms of human life.
We now need locks on our doors, and policemen in the street. Things change.
Nobody has to accept SMTP mail sent at them, like you don't have to have a letter box in your door.
plusnet unlimited fibre
80/20 - 2 Jun 14 - Sync at 28/Jul/17: 64,899/9,065 - G.INP & 3.3 dB SNRm
18 years of UK broadband since 1999 ntl:cable modem trial - Asus RT-AC68U and HG612 - BQM