I have used tools like these for nearly a decade and find them invaluable. My current favourite is Mailpit [1]. The “OG” I would consider Mailcatcher [2] from my Rails days [1] https://github.com/axllent/mailpit [2] https://mailcatcher.me/
The "OG" fake SMTP as a service would be Mailtrap [1].
[1] https://mailtrap.io.