Gmail was originally known internally by the codename Caribou before its public launch.
The project was developed by Google engineer Paul Buchheit several years before Google announced the service. For much of its early life, Gmail was kept relatively secret inside Google, with only a limited group of engineers working on it. As development progressed, Google employees began using it for the company’s internal email system.
Google announced Gmail publicly on April 1, 2004, as a limited beta release. The launch date led many people to suspect that Gmail was an April Fools’ joke, especially because its proposed storage capacity was dramatically larger than that of many competing services at the time.
Gmail remained in beta for years and officially left beta status on July 7, 2009. Caribou was an internal development codename, not a public predecessor that users could sign up for. That distinction is why answers such as Inbox or Mercury do not fit the question: they were not Gmail’s original internal project name.