What's the story behind Gmail's birth? Was it originally just an internal project?

Yes, Gmail indeed started as an internal Google project, and its origin story is quite legendary, full of the free-spirited, innovative engineering culture characteristic of early Google.

This story begins with a Google engineer named Paul Buchheit. Around 2001, the email system used by Google's internal employees was terrible—slow and difficult to use, leading to constant complaints. Especially the email search function; for a search company, their own email search being so awful was practically a joke.

Buchheit was actually working on other projects at the time, but he felt, "I've had enough; I need to build a good email system." So, he leveraged Google's famous "20% time" policy—which allowed engineers to spend one day a week on projects they were interested in, unrelated to their main job—and secretly began tinkering with a new email system.

He set several core goals for this project, which at the time seemed almost fantastical:

1. Fast! He used a then-new technology (later known as AJAX) that allowed web pages to update only the necessary parts without a full refresh each time. This made Gmail feel like a desktop application rather than a sluggish web page, a revolutionary user experience.
2. Search, not organize. He thought, "We are a search company, so why should users painstakingly create folders to categorize emails? Just give them a powerful search box, and they can find anything they want instantly—that's the Google way." This concept completely changed how people managed their emails.
3. Never delete emails. At the time, free email services like Hotmail offered only a few megabytes of storage, forcing users to constantly delete emails to free up space. Buchheit's goal was to provide enough storage so that users wouldn't even have to think about deleting emails.

This internal project, codenamed "Caribou," was initially a one-man show with Buchheit. Gradually, as the project demonstrated astonishing potential, more engineers joined. It was tested internally at Google for several years, with employees becoming the first users and testers, continuously providing feedback and suggestions for improvement.

The most dramatic part was its launch. Google chose April 1, 2004—April Fool's Day—to officially release Gmail to the public. When people saw the announcement of "1GB of free storage," their first reaction was, "Are you kidding? It's April Fool's Day!" Because at the time, mainstream email services typically offered only 2-4MB of storage, 1GB seemed like a fantasy, so many thought it was a Google April Fool's joke.

Furthermore, Gmail initially adopted an "invitation-only" system. You had to receive an invitation link from a friend already using Gmail to register. This scarcity instantly ignited social networks, with a Gmail invitation code even being sold for tens or hundreds of dollars on eBay. This was a classic case of scarcity marketing, and it also ensured that the servers wouldn't crash from too many users flooding in at once.

So, to summarize: Gmail originated from a brilliant engineer's exasperation with internal tools, born under an innovation-encouraging "20% time" policy. With its disruptive features like "search, massive capacity, and lightning-fast experience," coupled with a legendary April Fool's Day launch and a viral invitation system, it ultimately revolutionized the entire email industry.