Email has essentially converged on a patchwork ad-hoc net-wide implementation of a few of the proposals lampooned in the famous Slashdot copy/paste thing. Small businesses who are serious about getting their mail delivered pay what amounts to a delivery tax. The difference is it is not actually a tax, it is just a per-piece rate paid to a mailing service that keeps up with all the SPF records, feedback loops, blacklist monitoring, etc for us. However, considered from the perspective of the firm, it is essentially a tax, and it means that people paying a penny or two per email end up trustworthy. Everyone else is left in the email wild west, where they either have massive amounts of physical and reputational capital (Amazon et al) and get their mail accepted for free, or they're almost certainly trying to spam you (statistically speaking).

This is strongly related to strong centralization of email. I just had my 20,000th email submitted yesterday. Of those 20k, over 12k belong to just 10 domains. Even that overstates the diversity of spam squashing strategies, since most of the domains eventually use the same RBLs, etc.