We first covered Zimbra back in September 2005. Zimbra is an Ajax Microsoft exchange competitor with a webmail service that thousands of businesses and organizations use to handle email, contacts and calendaring. They also offer a great mobile solution. The core product is open source, and Zimbra has a higher end version that sells for $25 per person per year (with various discounts).

Since their launch they’ve grown. And grown. They had 4 million “paid mailboxes” In October 2006. Next Monday they will announce that they now have more than 6 million paid mailboxes over 1,300 customers, a growth of 50% in three months. Sixty percent of their customers are being serviced through resellers.

They have lots of help with the product, too. They’ll be announcing version 4.5 of their Collaboration Suite (which is already available). 6,300 developers and administrators have contributed to Zimbra. The open source version of Zimbra has been downloaded “hundreds of thousands” of times.

The company has raised $30.5 million over three rounds of funding from Benchmark Partners, Redpoint Ventures, Accel Capital, Sumitomo and Duff, Ackerman & Goodrich.