Marketing platform Salesforce.com announced today it plans to acquire ExactTarget, which sells e-mail marketing technology and services to such retailers as Best Buy Co. Inc. and The Home Depot Inc. for roughly $2.5 billion.

Salesforce.com will pay $33.75 per ExactTarget share, 52.7% higher than its $22.10 closing price on Monday. Both companies’ boards of directors have approved the deal.

The move is Salesforce.com’s latest effort to use acquisitions to round out its marketing offerings. The vendor last year bought social marketing platform vendor Buddy Media for $689 million. Two years ago it acquired Radian6, which helps marketers monitor conversations on social networks.

In adding ExactTarget to Salesforce.com’s tools, the vendor will offer marketers a wide-ranging portfolio that enables them to manage e-mail, social media marketing and mobile campaigns. That broad range of services will make the vendor more attractive to marketers, says Marc Benioff, chairman and CEO of SalesForce.com.

“The addition of ExactTarget makes Salesforce the starting place for every company and puts Salesforce.com in the pole position to capture this opportunity.”

The move comes as research shows that digital marketing spending is rapidly growing. Research firm Gartner Inc. estimates that by 2015 consumer technology companies will have moved about one-third of their traditional marketing budgets to digital.

ExactTarget serves 36 retailers in the Internet Retailer Top 500 Guide and 24 in the Second 500. Salesforce.com, including Buddy Media and Radian6, serves 8 retailers in the Internet Retailer Top 500 Guide.