Before Mailbox even launched, it had already built up a massive waiting list, making it an invite only service when it was first born so the servers weren't overloaded. Dropbox ended up buying the service which of course aided the app massively as Dropbox know a thing or two about servers and handling millions of users.
Mailbox at first only launched with Gmail support, but today it has been updated to now include iCloud and Yahoo email support. iCloud support may be the biggest announcement here as it's been the most requested feature since launch.
The app still remains free and is available for both iPhone and iPad. All it needs now is an Android version. The developers have said they are working on an Android version, but this was all the way back in May.