So you need to test and debug email notification templates while working in your local environment. You still need to verify mail is being sent and inspect the email contents. You might create a few accounts with email addresses you own, which I guess works on a small scale. But let's say you need to test bulk mail delivery, or maybe you're working with a preexisting member list of thousands of accounts. Even though you're working on a local server, the email addresses are real and if you mistakenly trigger a bulk mail function, you're going to end up with some very confused users. Of course you're already bootstrapping your EE setup with the Focus Lab conifg overrides right? We'll be using MailTrap.io to isolate and display sent emails from our dev environment.