Lost in the news of Zimbra’s release of version 6.0 of its collaboration suite is the importance of one very big number: 50 million. That’s how many paid mailboxes Zimbra claims now, a number that puts it within spitting distance of IBM Lotus Notes (approximately 145 million paid mailboxes) and Microsoft Exchange (approximately 175 million paid mailboxes). Whatever the truth to rumors that Zimbra is up for sale, Zimbra is an appreciating asset for Yahoo, not a depreciating one.

For one thing, it’s important to consider just how explosive this growth has been. In January 2009, Zimbra was at 20 million paid mailboxes. By March, that number had jumped to 40 million paid mailboxes, perhaps a consequence of Comcast mailboxes coming online. (Comcast chose Zimbra to power its Web mail service in 2007.)

That’s a lot of growth in a very short period of time. Zimbra appears to have more than doubled its business in the past year.

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