Only one message in Mail in fullscreen?

How is it that you can only have one new message window open in Mail if it's in fullscreen?

Mail is perfect for fullscreen - all that space - but you can only start one new message? That's a step backwards.

It's fine when Mail isn't in fullscreen.

Also, if you create a couple of new messages when you aren't in fullscreen, then switch to fullscreen, those messages only show up in the original desktop (not in the fullscreen one) and you get to them with the Window menu.

I just cannot now use Mail in fullscreen as I'm used to having lots of messages open at once (very busy, you see).