Comment by philipstorry

SMTP won because it was simpler, but it's probably good to look at why it was simpler.

SMTP handled routing by piggybacking on DNS. When an email arrives the SMTP server looks at the domain part of the address, does a query, and then attempts transfer it to the results of that query.

Very simple. And, it turns out, immensely scalable.

You don't need to maintain any routing information unless you're overriding DNS for some reason - perhaps an internal secure mail transfer method between companies that are close partners, or are in a merger process.

By contrast X.400 requires your mail infrastructure to have defined routes for other organisations. No route? No transfer.

I remember setting up X.400 connectors for both Lotus Notes/Domino and for Microsoft Exchange in the mid to late 90s, but I didn't do it very often - because SMTP took over incredibly quickly.

An X.400 infrastructure would gain new routes slowly and methodically. That was a barrier to expanding the use of email.

Often X.400 was just a temporary patch during a mail migration - you'd create an artificial split in the X.400 infrastructure between the two mail systems, with the old product on one side and the new target platform on the other. That would allow you to route mails within the same organisation whilst you were in the migration period. You got rid of that the very moment your last mailbox was moved, as it was often a fragile thing...

The only thing worse than X.400 for email was the "workgroup" level of mail servers like MS Mail/cc:Mail. If I recall correctly they could sometimes be set up so your email address was effectively a list of hops on the route. This was because there was no centralised infrastructure to speak of - every mail server was just its own little island. It might have connections to other mail servers, but there was no overarching directory or configuration infrastructure shared by all servers.

If that was the case then your email address would be "johnsmith @ hop1 @ hop2 @ hop3" on one mail server, but for someone on the mail server at hop1 your email address would be "johnsmith @ hop2 @ hop3", and so on. It was an absolute nightmare for big companies, and one of the many reasons that those products were killed off in favour of their bigger siblings.