Yahoo said Monday it was buying open source email and calendar service Zimbra for $350 million.

The deal will help Yahoo leverage its popular Yahoo Mail service as the company struggles to compete with Internet giant Google and social network sites such as MySpace and Facebook.

Founded in December 2003, Zimbra provides white-label email services to ISPs, universities and small- to medium-sized businesses. In March, the company announced it had 6 million paying customers, many of whom had chosen Zimbra as a cheaper alternative to Microsoft’s Exchange platform.

Zimbra recently landed a major deal to provide email service to subscribers of Comcast, the largest U.S. cable operator. Under the agreement, Zimbra could also provide visual voicemail services and calendar function tied to television schedules and on-demand content. Yahoo already has a deal to provide display advertising for Comcast.

Zimbra, backed by Benchmark Partners, Redpoint Ventures, Accel, Sumitomo, and Duff, Ackerman & Goodrich, has raised a total of $30.5 million.

Buying Zimbra will allow Yahoo to build on its strength in email. It has 250 million users on email to rival Google’s approximately 50 million. Yahoo also recently released a well-received new version of Yahoo Mail that lets users send instant messages within the email program and send text messages to mobile phones.

A key weakness for Yahoo is its lack of a cohesive social networking service that rivals MySpace or Facebook. The company is now working on a new service called Yahoo Mash, but Yahoo president Sue Decker in July pointed to Yahoo Mail as a promising social networking opportunity.

Ms. Decker called it “one of the Web’s largest dormant social networks and one that we are aggressively pursuing ways to activate.”

It remains to be seen whether the Yahoo Mail audience will be interested in social networking of the sort that runs wild on MySpace and Facebook.