How do I configure the Windows 10 Mail app as the default MAPI client

The Send to ... Mail Recipient uses the MAPI\Send Mail association, or the Send mail command.

The Microsoft Mail app only contains an association for the MAILTO protocol, so you can't set it as a recipient of the Send mail command.

It's the same as windows 7 and 8, which both also shipped with no mapi support. You always have to install your own mapi client now. I think they want it to die, especially shipping his many major versions without such support. Vista was the last to come with it out of the box.