I worked on very early versions of email on the ARPANET, actually. And the problem that I was trying to solve was that there were a lot of networks—the ARPANET was just one of them—and people wanted to send email between these networks.
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When I started, of course, there was no capital‑I Internet. There were, however, eventually a bunch of smaller networks that didn’t talk to each other, and I decided that they needed to talk to each other at least for email.