Aqua Mail is absolutely the best email client for newer Chromebooks that support Android apps (1-2 years old or newer, in most cases). When Google first supported Android on Chrome OS there were issues with many apps, including Aqua Mail, but now all is good. If you want an email app that reliably keeps the amount of history you tell it to, between sessions, this does it best, and it has the look and feel of what most people have been used to, if your history is with Windows. I've used Aqua Mail for years on Android, and it is a tremendous enhancement to Chrome OS.