Commercial and consumer email was not always federated.
In the early 90s, businesses had local area network emails like Banyan Vines, MS Mail, or Novell NetWare.
In private life, people used online services like Compuserve, AOL, or local BBSes.
Internet email was an optional feature of some systems, usually difficult to get or use.
There were competing standards for messaging, like X.400.
Then, it all became SMTP-enabled.
It all happened very fast.
Gradually then all at once.
@evan what does SMTP have to do with centralization? Or do you mean it was just the standard that became dominant.
@mcneely it was the standard that brought all those consumer and business networks together.
Metcalfe's Law made the combined email internetwork much, much more valuable to everyone.