This feature is currently in Beta. Switch to Beta.
Update your Wavebox to the Beta Channel, and click on any email address to reveal a shiny new email picker, showing a list of ALL your webmail addresses. Just click on the one you need to launch a compose window ready for your message.
Here's how to get it working in just two simple steps.
Pick your Default Browser and Email Client.
Start by setting Wavebox as your default browser and default mail handler. You can do this easily in Settings > Default Browser & Handlers. Click on the 'Make Default' button for each.
Click and Compose.
Find an email address in any app on your OS, or in Wavebox, to see the email picker modal in action. Choose the account you need to open a new compose window for that account.
There are also options to copy the address to clipboard if needed, and to open in a new app window (not signed-in).