I wasn't going for Outlook when I got the news that Mailbox is shutting down.
I've been using Mailbox on my iPhone4 and continue to do so when I've move to OnePlus One.
To my surprise when I search the Play Store for mail the first result was Outlook. I've not use outlook for over 10 years, and I never looked back.
The reason I'm not using Gmail nor Inbox is that they do not support multiple accounts in one view. I have both my personal and work email, both hosted by Google, and I don't want to keep switching between the two accounts.
Outlook is fast, clear and support multiple accounts in one view.