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.
In the space of a year, then, Zimbra has roughly doubled its customer base. Let's correlate this to sales.
The company(and was on track to do just that as of September 2007, when Yahoo! acquired Zimbra), the year that it probably bumped up against 9 million paid mailboxes ($2.22 per mailbox). It did $6 million in 2006, the year that it had 4 million paid mailboxes ($1.50 per mailbox).
If my math is anywhere near accurate (not a good assumption, as Marc Fleury will tell you :-), the value to Zimbra of each mailbox may be growing at a 48 percent clip year over year, while the number of paid mailboxes roughly doubles.
So, if we assume the value of the mailboxes rises to $3.29 in 2008 (Improved brand and product causes customers to spend more money with Zimbra), and paid mailboxes to increase to 20 million, then Zimbra ends 2008 with roughly $66 million in sales.
Even if we assume the value of the mailboxes doesn't rise at that rate, but stays flat (a reasonable bet in a down market), Zimbra still ends up doubling its sales to $44 million in 2008.
Pretty impressive growth, especially in a market where the top-two providers, IBM and Microsoft, have such dominant positions. It points to the value of tight execution and an open-source lead machine.
Disclosure: After months of using Zimbra, I am an unabashed fan leaving me hopelessly biased in its favor.