It really really hasn't been. Email is just an enormously massive ecosystem with tremendous amount of variance that anything new will take a long time to adopt

It really really hasn't been. Email is just an enormously massive ecosystem with tremendous amount of variance that anything new will take a long time to adopt and a lot of convincing and proving. Just look at how far the deployment of something as essential as DMARC is, it's sad. That's not some big vendor's fault, it's all the small legacy cruft nobody wants to clean out as it might break things.