In rolling out their popular new email client, the developers behind Mailbox played it about as safe as anyone possibly could. A long waiting list stands between iPhone owners and access to the sleek, innovative app. But today it's become clear that this phased approach has failed to shield Mailbox from complications. For the last several hours, Mailbox has been down, delivering a "having trouble connecting" message to its rapidly-expanding crowd of users. On Twitter, developers are attributing the hiccup to "an unusual server problem" and have promised a timely fix.