Exactly 15 years ago, on April 1, 2004, Google surprised many with the launch of Gmail, its own web-based email client. With 1 gigabyte of free storage, conversation-based grouping of messages and enhanced search features, the service was superior to most, if not all competitors at the time, making its eventual success look inevitable in hindsight.

With 1.5 billion users, Gmail has grown into one of Google’s most successful products and one of the most popular email clients in the world. According to data published by email analytics firm Litmus, Gmail is only beaten by Apple’s native iPhone email app in terms of usage. According to Litmus’ analysis of 834 million email opens worldwide, 26 percent of those emails were opened within Gmail, while 28 percent were opened in the iPhone email client.