Holy moly! I was totally shocked when I saw K-9 Mail pop up in BlackBerry World. The hugely popular open source Android email client works beautifully on BlackBerry 10 and its free! I am still surprised that BlackBerry allowed a third party email client into BlackBerry World (which used to be against their policies). Still they chose the best email client to allow through.
If you want to have a separate email address on your BlackBerry 10 device that is not in your hub (say a spam address) I highly recommend K-9 Mail. I would also recommend picking up K-9 Mail in BlackBerry World ASAP in case BlackBerry changes their mind and removes it!