updated 03:21 pm EDT, Fri April 26, 2013
Support for non-Gmail accounts also a possibility
An iPad edition of Mailbox is in development, as are Android and desktop versions, tweets by the app's development team indicate. The iPad app is said to be "coming soon," while the Android and desktop apps are on a longer-term roadmap. Support for other platforms may also be possible.
Mailbox is presently iPhone-only, and tries to improve on iOS' default email client through things like a gesture system and the ability to have messages return at a more convenient time. The development team was recently bought out by Dropbox, which has allowed them to remove an invitation system that restricted Mailbox access. The app currently works only with Gmail accounts, but support for hosts like iCloud "may be added as we grow," Dropbox says.