What are some defunct protocols or systems (X.400, UUCP, Lotus Notes)?
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Before SMTP became the standard, several email systems competed for dominance. Each had its own routing logic and addressing format, and they all eventually hit the same wall: they couldn't scale beyond small networks.
UUCP (Unix-to-Unix Copy Protocol) was the most notorious for its complexity. Messages were routed manually using what's called a bang path. An address looked like this:
host1!host2!host3!user
The sender had to know the exact route the message would take across machines. Each exclamation mark (bang) represented a hop from one server to the next. If a machine in the chain was offline or misconfigured, your message failed. No automatic rerouting. No global directory. Just manual path plotting for every single email. It worked for small academic networks but became unmanageable as email grew.
X.400 was the official international standard backed by telecom companies and governments in the 1980s. Unlike UUCP's anarchy, X.400 had strict hierarchical addressing. An address included country codes, organization names, and personal identifiers. Think of it as the postal service's attempt at email. It was technically sophisticated but bureaucratic and expensive. Internet email (SMTP) won because it was simpler, cheaper, and didn't require permission from telecom regulators to set up.
Lotus Notes (now HCL Notes) wasn't exactly a protocol, it was a proprietary enterprise collaboration platform with email built in. Companies used it for internal messaging, but Notes email lived in its own walled garden. To send Notes email to the internet required gateway software that translated between Notes' internal format and SMTP. Many large corporations ran Notes through the 1990s and 2000s. Some still do, though most have migrated to Microsoft 365 or Google Workspace.
Why SMTP won: it didn't require central coordination, didn't charge licensing fees, and used simple domain-based addressing (user@domain.com) that anyone could understand. By the mid-1990s, the internet had made UUCP obsolete, X.400 had lost the standards war, and Lotus Notes became a legacy system that companies migrated away from as soon as they could afford the disruption.
So if you're curious how email routing actually works today (spoiler: no bang paths), check out how MX records handle routing automatically.
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