Email has functioned as a universal software interface since the 1970s because it solved a fundamental problem: how to let users interact with programs without building separate apps for every operating system and platform. When computers ran on incompatible systems—different Unix variants, Mac, Windows—developers realized email worked everywhere. It required no signup, no download, no account creation. A user could send a message to a machine's email address, the server would parse the text, execute commands, and send back results. This turned email into something that looked human but functioned as an API, making it the longest-running consistent software interface alongside terminal commands. The 1985 Netlib system demonstrated this concept at scale. Jack Dongarra and Eric Grosse built what amounted to an '80s App Store distributed entirely through email. Researchers could email requests for mathematical software—"send index" to list available code, or "send quadpack/dqag" to retrieve specific routines—and the server would respond with the requested files within minutes. By two years after launch, Netlib was handling nearly 4,000 downloads monthly. The system worked because email was the only ubiquitous communication standard available; it needed no new infrastructure, no platform-specific builds, no website. Users preferred it to hunting through their own machines. The pattern repeated throughout the '80s and '90s. FTPmail let you fetch files from FTP servers via email. MCI Mail sent faxes through email. IMDb operated entirely by email in its early days—you'd query the database with messages like "title: Casablanca" and get back results. Ringo delivered music recommendations over email. Even as web apps and SaaS emerged, email remained the fallback interface because it fit how people already worked. No slow browsers, no app installation, no new learning curve. Just open your email, send a message, get results back.