How is email 'effectively' centralized? There are tons of providers, you can even host it yourself. The only thing that might be somewhat centralized are spam filtering algorithmns that are shared, but many aren't.
By having big provider turn up the spam detector for small actors. It's currently common to have a perfectly set up mail server, with verification and reputation and longevity… and still have Gmail flags the emails that come from it as spam.
Also, forget about sending email from home. Email sent from residential IPs are instantly deleted, and not even sent to the spam folder, by big providers (Hotmail makes it an explicit policy, and bounce the email right back). You have to at least relay the damn mail through a non-residential IP.
A week of troubleshooting and we discovered GApps didn't like emails coming from Linode IP addresses.
A sprint and a half later we had rewritten our email functions to use sendgrid. An extra expense, extra dev time burned. Becaus we had done everything right but upstream it still wasn't good enough.
The world of commerce has a variety of mechanism to ensure credibility and accountability (signed contracts, security deposits, identification systems, and so on). Extended validation certificates is one manifestation of this in the digital world.
A decentralized service that relied on a consolidated reputation system might be able to provide a framework for managing bad actors.
Once you have a strong centralized reputation system, you are pretty close to having re-centralized your decentralized system once again. Especially in a context where a relatively small number of systems send and receive most of the email to begin with.
Sorry, but this hasn't been my experience at all. I see this claim all the time, and, obviously, ymmv, but I've been using residential IPs for ~10 years with no issues.
> 553 5.7.1 [BL21] Connections not accepted from IP addresses on Spamhaus PBL
> Dynamic or residential IP addresses as determined by Spamhaus PBL (Error: 553 5.7.1 [BL21])
> The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer's use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.
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I guess you're lucky enough that your IP adress has not been tagged as "residential" by Spamhaus.
I found the fine article we are discussing to contain a great link explaining emergent centralization, including of email. Perhaps you had a different experience.