Worked on improving the stability and test coverage of the PHPMailer/PHPMailer repository by addressing a critical issue that could cause infinite recursion or crashes when the Mailer property was null or empty. Applied a test-driven development approach using PHP and PHPUnit, introducing a guard to prevent the send method from executing under invalid conditions. Enhanced the reliability of mail delivery by adding comprehensive unit tests, including a data provider to cover edge cases and updating the EmptyMailerTest with new scenarios. Focused on backend development and software testing practices, resulting in fewer crash-related incidents and more robust PHP code for the project.