Well that was an interesting puzzle!
If I create a GB project on my iPad, save it into iCloud, then open it through iCloud Drive on my Mac it all works fine. I then tried forwarding the project via email, dragged the project from the email to the desktop and tried to open it. GarageBand tries to open, prompts you to create a new (presumably GB for OS X) project to copy into, and then fails. Looking at the file sizes of the email attachment compared to the copy downloaded via iCloud drive gives a clue, they're not the same. I renamed the .band on my desktop with a .zip file extension, double-clicked it, and out popped a new .band file of the correct size. This one opens in GarageBand! 🙂
I suspect the same approach might be necessary if you export to iTunes, copy that out using iTunes File Sharing, and then try to mail the .band file. I don't have time right now to test.
tt2