One application that Yahoo will make available creates a link between Flickr and Yahoo Mail. The service looks at the subject line of an e-mail and searches Flickr for photos related to that word, such as "party."
The company envisions a whole list of applications that can be built using mail.
For example, people can find ways to access e-mail from different mobile clients or to combine social networking features and multimedia with mail, said John Kremer, vice president of Yahoo Mail.
The company is also offering financial incentives to developers who bring in paying mail customers.