E-mail is already decentralized. Anyone can send and receive an e-mail with only having network access as a barrier, and you only need a typical modern computer to send millions of them.
Mailchimp's value proposition is that by becoming an authority over a chunk of distribution using their brand, anything they send carries a badge of trust since they promise it isn't malicious. Their e-mail templates are a nice bonus, but I don't think that's the only reason some people use them. That mailchimp origin server is a keycard through a bunch of gatekeepers propped up as sentries against spam.
In essence e-mail is totally decentralized and trust is exchanged amongst members of ad-hoc federation of parties with a vested interest of having non-spam email get through the internet on both receiving and sending ends.
Is there a way a blockchain would make this market-based emergent system more efficient?