Mailbox, the popular email client for iPhone, is finally doing away with its reservation system and accepting new signups immediately. The app launched in February to widespread acclaim, but Mailbox's staggered rollout scheme proved to be an annoyance to many users. It also failed at its fundamental purpose of preventing the app from experiencing downtime. At its peak, the reservation queue swelled to more than one million users. That massive popularity led Dropbox to scoop up Mailbox to the tune of $100 million, promising to keep the product alive and evolving in the future. Ten weeks of "around-the-clock" work combined with whatever resources Dropbox is pumping into Mailbox seems to have done the job, with the app now scaled to deliver up to 100 million messages per day. Considering there's no waiting involved, now could be a good time to test out the Mailbox's unique approach to inbox management.