I often use a bluetooth keyboard with my tablet and use keyboard shortcuts everywhere. In the Gmail app and in the web versions of both gmail and inbox, the shortcut to send a mail you're writing is ctrl + Enter. This shortcut doesn't work in the Inbox app for Android, but it seems there is a shortcut, because I've accidentally sent emails before finishing them, but I don't have a clue what triggered the action. A Google search didn't give me an answer to this question, so I'm asking here. What is the shortcut to send a message went composing a message in the app version of Google Inbox?
According to Keyboard shortcuts for Gmail, Ctrl + Enter is the correct shortcut for sending emails on Android (just as the other version stated in your question).
If you plug an external keyboard into your Android device, you can use the shortcuts below. This works with most kinds of external keyboards.
Here are the available shortcuts.