Do away with the PSL and Org Domain entirely; just walk the tree

[seth@…]

When DMARC was first developed, there was concern about DNS load and needing to minimize DNS lookups. Operational expertise now shows that this is no longer cause for concern.

Short circuiting a tree walk has led to many issues, like a reliance on the PSL, complicated algorithms for Org Domain discovery, many types of domains (PSDs, per ​https://tools.ietf.org/wg/dmarc/draft-ietf-dmarc-psd/) being unable to utilize DMARC even though they wish to, and larger organizations (such as universities and governments) that are comprised of sub-organizations that use subdomains having material problems getting everything authenticated.

All these issues disappear, and DMARC becomes a lot simpler conceptually, if DMARC simply walks the DNS hierarchy for the exact sending domain down to the TLD until it finds a DMARC record, and stops.