You could use (a variant of) SMTP as a transfer protocol in Bitmessage ignoring target host discovery and using the p2p network for relaying, and then it would be decentralized... but without them SMTP is not much besides the envelope part, which is pretty meaningless.

The great thing about Bitmessage is that it replaces DNS, target host discovery and single sending/receiving points.

SMTP relies on single relay points, while Bitmessage relays through the whole network (regardless of who's online or not).

Bitmessage's architecture actively avoids users lumping in a single provider since each user is his own provider by design.

Also: SMTP relies on mail servers and ties addresses to them. In Bitmessage, your address belongs to the whole system. You can't switch from an SMTP provider to another while keeping the same address. That's a single point of failure.

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Imagine a perfect scenario where everyone on the internet has its own mail server and nobody uses DNS for target discovery because they send emails directly to IPs (e.g. target@10.10.10.10). What if your IP changes to 10.10.10.11? Are you going to update your address everywhere? What about people with dynamic IPs? What if I don't want to give you my IP? You wouldn't have a mail address you could publish!

How did we solve these problems? Using centralized DNS and mail servers. Which is what we want to avoid in the first place.

So SMTP isn't centralized by design, but for it to be comfortable, it has to be centralized.

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And Bitmessage solves even more problems:

Let's say your ISP or your government wants to isolate you from communications.

You publish target@10.10.10.10 as your email. Your government finds it out and tells your ISP to close your server at 10.10.10.10.

Now you won't receive communications. Single point of failure!

You can change your address (which is a burden in and of itself) but then you have a single point of failure again which is as easy to cut you off from.

You can't do that in Bitmessage because everyone has and can relay your email, whichever their IP or yours is. They'd have to control and cut you off from all relay servers, which is everyone on the network as opposed to email where they have to cut you off from a single server.

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Okay, so for SMTP to be decentralized we have to strip email down so it:

- Wouldn't rely on DNS which is centralized.

- Wouldn't rely on target host discovery, which creates single points of failure (i.e. addresses weren't tied to single servers and could "float" in the network).

- Would broadcast your messages to all their known relays (which would in turn relay to all their relays, which would in turn... ad infinitum) so they can't cut you off the network.

Then SMTP would be decentralized.

But I have just described Bitmessage!