DoubleClick Inc., New York, said Monday that it has entered into a strategic partnership with Topica, San Francisco, to provide e-mail services to its customers. The move means Topica will be sold alongside DoubleClick products, such as banner advertising network services and the targeted Sonar Network. Topica, currently delivering 250 million e-mail messages monthly, fills a void in DoubleClick's services. Amid a growing demand for e-mail services that run along with banner advertisements, DoubleClick, New York, introduced in December management software that allows advertising clients to send e-mail to customers requesting additional marketing materials. However, it couldn't deliver a ready-made list of prospects without gleaning data from its advertising database--a potentially risky move in the heightened federal interest in ad-network tactics. With an outside partner, who has already gained customer consent to receive e-mail, DoubleClick will be able to resell lists of business prospects.