Seems that migration isn't that easy...
But CERN doing it's own thing w.r.t. e-mail, as first step would say is questionable. There are a fair few mail providers out there and does it really add value to CERN's core activity by bringing it in house? It's not just pumping mail around, it's supporting the clients, browsers, scanning for spam/malware etc that all turn into a lot of work.