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Let's say you're working on a web$-$based email application. The application should be able to register users, allow them to log in$,$ send e$-$mail, receive e$-$mail, create folders and store messages within them, search e$-$mails, and allow the user to customize their account settings. Performing an extensive series of tests to ensure that every function of the application is working, from the end$-$user perspective, is an example of $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ testing.