Mailspring is a free mail client for Mac, Linux, and Windows (a paid Pro version is also available). Mailspring's user interface (UI) is open source (GPLv3). It is built on a plugin architecture and is designed to be easy to extend.

I have started using Mainspring on macOS recently, and since I find keyboard shortcuts important when using an email client efficiently, Mailscript's highly configurable keyboard shortcuts are a big plus. For work email, I have to use Microsoft Outlook, and I prefer to have the same keyboard shortcuts for both email clients. In Mainspring, you can modify a set of keyboard shortcuts when you go to the shortcuts section in settings. But you can also choose «Edit custom shortcuts» to open the keymap.json file and edit it directly. This gives you the possibility to edit and add more keyboard shortcuts. A simple example of such a file where the keyboard shortcut ⌘N is set to create a new message:

{
    "application:new-message": "mod+n"
}

When I started to edit and add keyboard shortcuts to this file, I wanted a complete list of possible functions (menus) that could be accessed with keyboard shortcuts. The best source I found was the source code of Mailspring's UI. You can find all the menus of the email client in the app/menus/ folder of the source code, one file for each supported OS: darwin.js (macOS), linux.js (Linux), win32.js (Windows). For example, under the «Thread» menu on macOS, you'll find this line:

{ label: localized('Move to Folder') + '...', command: 'core:change-folders' },

If I want to add a keyboard shortcut to move a message to a folder («Move to Folder…»), the line above from the source code identifies the function core:change-folders to be the function I am interested in. I can then update the keymap.json file to add ⇧⌘M as a keyboard shortcut for this function:

{
    "application:new-message": "mod+n",
    "core:change-folders": "shift+mod+m"
}