Presumably to stem the crushing demand, Mailbox restricts the number of new accounts that can be activated at a time.

CNN: Dropbox acquires buzzy app Mailbox

The Mailbox app, which will remain a stand-alone app after the acquisition, features a number of swipe gestures that allow users to quickly plow through email messages.

FORBES: Showing Broader Ambitions, Dropbox Acquires Mailbox