Just got the new ZDC build 1510 and while trying to break it I noticed that I couldn't share mail folders with external guests and public. Checking back with the web client, I see the same limitation there. (ZCS 5.0.12)
All other types of data seem to be share-able (view only) with external guests and public. I'm kind of agnostic whether sharing mailboxes this way would be a useful feature at all, but I'm mainly curious as to what the expected behavior is. I really thought the capability used to be there (at least in the UI, not sure if it really worked).