"I was on Zimbra's site today looking for the latest update to its excellent Desktop product, and came across the news that open-source Zimbra now has 11 million paid mailboxes. This might still be small potatoes compared to IBM's Domino and Microsoft's Exchange, but it signals tremendous growth from Zimbra, as a quick Google search confirmed.
"Back in October 2006, Techcrunch reported 4 million paid mailboxes for Zimbra. By January 2007, the number had jumped 2 million to 6 million paid mailboxes..."