"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..."