Is email still people-centric? Looking at the 25 latest messages of my inbox, only 8 are from real persons. The rest comprises 5 newsletters and 12 notifications.
Close friends I contact via SMS mostly, and other friends I contact maybe twice every 6 months.
Maybe my email usage is unusual, I don't know. Still, Unibox's market is a niche. Good luck to them though. It's always interesting to see new approaches towards a medium as old as email.