Yahoo on Wednesday is expected to release software that will allow third-party developers to write applications using Yahoo Mail. The move to open up the application programming interfaces (APIs) to Yahoo Mail is meant to encourage Web developers to build applications that use the mail service. The company expects these third-party "mashups," hybrid software that combines content from more than one source, will bring in more customers. "There is tremendous potential for additional usage of Mail. By providing more user experiences, we believe we will increase engagement and utilization of the platform," said John Kraemer, vice president of Yahoo Mail.