zimbra.bmpZimbra, the open source messaging software company, has just announced that it has sold four million Zimbra mailboxes, an impressive milestone for the three year old San Mateo company.

Zimbra, you’ll recall, gives you an email platform that implements the latest AJAX majic. It started last year by letting you do things like pull up Google maps by scrolling your mouse over an address written in the e-mail, or pop up your calendar when you mouse over a date in an e-mail, or a day of the week — avoiding the need to clunkily switch back and forth from your e-mail and your calendar.

It is one thing to look good, quite another to execute. Since then, it has come out with Zimlets, which let developers do even more. This has proven unexpectedly popular with Internet service providers, Satish Dharmaraj told us today. “I can’t believe what we sold this quarter,” he said. He wouldn’t provide revenue numbers.

He said service providers are hosting Zimbra’s email service for individuals and small and medium sized companies.

The providers like Zimbra because it is “skinnable,” meaning it can be tailored for a consumer feel, or for a business feel. Service providers can choose to monetize the email service by running Google Adwords or Adsense, for example. Zimbra gives them a way to make a cut if a user clicks an ad from within their email to buy an iTunes song for 99 cents, or a book from Amazon. Like Gmail, Zimbra indexes every word within an email, and so knows what is being written, and can offer relevant advertising or other services — depending on what the service provider wants.

Before and after Zimbra’s launch, there have been numerous companies seeking to improve on Microsoft or other email services.

Zimbra’s product is compatible with Microsoft Outlook and other popular e-mail platforms, such as Apple Mail. Zimbra really runs the back-end of the e-mail service, making it a competitor to Microsoft’s e-mail server offering, called Exchange. But Zimbra can keep the familiar “front-end” part of Microsoft’s e-mail platform, which users interact with, called Outlook.

Zimbra has been selling its high-end “enterprise” mailboxes at $28 a pop, but these are meant to compete with Exchange and have the bells and whistles. Most of Zimbra’s uptake has come through service providers serving individuals, however. Those service providers can have up to 20 million users, and Zimbra gives them a major discount based on volume. (In other words, you can’t multiply 4 million by $28 to get Zimbra’s revenue; not even close.)

Besides service providers, Zimbra adopters have included dozens of universities and other companies, such as Digg.com and Times of India (the announcement lists many more). As noted before, Zimbra has raised at least $30 million from Benchmark, Accel, Redpoint and others over three rounds.

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