In the early 90s I implemented a gateway between Novell email and X.400. What amused me the most was X.400 specified an exclusive enumerated list of reasons why email couldn't be delivered, including "recipient is dead". At the X.400 protocol level this was a binary number. SMTP uses a 3 digit number for general category, followed by a free form line of text. Many other Internet standards including HTTP use the same pattern.

It was already obvious at the time that the X.400 field was insufficient, yet also impractical for mail administrators to ensure was complete and correct.

That was the underlying problem with the X.400 and similar where they covered everything in advance as part of the spec, while Internet standards were more pragmatic.

Who can forget addresses like "utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!rms@mit-prep"

1. smtp predates dns. or really even most of the internet. It was originally designed to work over uucp.

2. early smtp used bang paths (remember those) where the route or partial route is baked into the path.