The acquisition will run Yahoo a significant chunk of change, at around $350 million.
If you've ever worked in a Windows environment at a workplace, you probably have dealt with Exchange. It holds a high place in enterprise e-mail, and is strongly rooted in the companies it exists in. Zimbra is one of the few firms that could actually give Exchange (and Microsoft) a run for the money, providing some actual competition for companies looking for Exchange alternatives. The biggest features offered by exchange, such as automated calendaring and advanced user management features, are things Zimbra has been tackling head on.
What's Yahoo's angle in this? Their core business focuses on search and advertisement. It seems to me like they might be chasing after both Google and Microsoft in this, or perhaps giving themselves more leverage to compete against either. Enterprise e-mail doesn't exactly seem like Yahoo's forte.