I started my career at France Telecom's R&D lab in Caen, Normandy. They had their own home-grown X.400 email client, and even though they could have set up a SMTP server for free, they deliberately chose to MX to a paid SMTP to X.400 gateway out of OSI ideology.

It was complete garbage.

Another lab of theirs proudly made a Winsock that would use ATM SVCs instead of TCP and proudly made a brochure extolling their achievement "Web protocol without having to use TCP". Because clearly it was TCP hindering adoption of the Web /s

The Bellhead vs. Nethead was a real thing back then. To paraphrase an old saying about IBM, Telcos think if they piss on something, it improves the flavor.

One of the jobs I had applied out of college was to lead Schengen's central police database (think stolen car reports, arrest warrants etc) which would federate national databases. For some unfathomable reason, they chose X.400 as messaging bus for that replication, and endured massive delays and cost overruns for that reason. I guess I dodged a bullet by not going there.