[tinco]

I think they're talking about GPG, not SPF/DKIM/DMARC.

Which is a risky thing to do, because most people don't associate GPG with positive feelings about well designed solutions, but they're right in that it works well, solves the problem and is built squarely on top of email.

The reason that it's not generally well received is that there's no good social network for distributing the keys, and no popular clients integrate it transparently.

[Avamander]

In this case GPG, DKIM and even S/MIME are on equal standing. Validity can be checked only on reception because there's no validity stapling mechanisms.