[jrockway]

The underlying problem is that SMTP only has a few error codes, and so there is no way the MUA can understand (in a standard way) why the MTA rejected the message. HTTP ended up having richer errors, and so clients can provide nice messages if they feel like it. (IE does this for 404s and 500s, though it's not a nice message and it seems to upset most web developers.)

[autarch]

And an HTTP server can return a custom body for most errors, so the server can provide even more info if it wants.

[jrockway]

SMTP MTAs can also "provide more info if it wants", but that's what the article considers a problem. What the author of the article wants is a way to dumb down error messages (or, less cynically, automatically correct errors), which is not easy with free-form responses. Imagine a web page that returns a page that says "404 Not found" with a 200 status code. Brokenness ensues.