PostPath, a Mountain View start-up that sells a corporate email server, one of several companies trying to take on Microsoft, has just raised more cash.

It has raised $15 million in a third round of funding, led by JAFCO, and including existing investors Matrix Partners and Worldview Technology Partners.

It wants to take market share away from Microsoft’s Exchange service, by making its Linux-based server compatible with Exchange, so that companies can use PostPath without having to make major changes.

The four-year-old company has now raised $30 million.

Other competitors include Scalix and Zimbra.