Abusive, SPF is plenty enough unless you cannot map the domain with the right IPs due to DNS trickery (rotation, etc), then you would need an IP agnostic way to do some checks, hence the cryptographic DNS based signature.<p>That said, with no-DNS email addresses, SPF comes for free (alice@[x.x.x.x] bob@[ipv6:...]).<p>Namely, if SPF does pass, cryptographic DNS based signature mecanisms are excessive and must not be used to score.