MailCatcher runs a super simple SMTP server

What is the intended or typical use for something like this? Testing other applications that send email comes to mind, or am I missing some other uses?

This exactly. You run it in your dev environment so that your application can send email and you can view it, without actually sending real emails (that might accidentally go out to real people if you aren't careful).

Yep. We use this locally as the default smtp server so we can see how our email templates render

We heavily use it for end-to-end testing. The API is integrated into our Playwright tooling, so we run e2e tests that verify entire user flows including email "steps" - such as the new user registration flow ("Open landing page - click through to registration - fill out and submit form - find confirmation email in Mailpit - click link - log in with new user credentials").

I use it with docker-compose.yml in Rails and Django applications to catch all emails coming out of a local development copy and be able to confirm they are sent as well as to troubleshoot the layout of those email templates.

I use it for testing and local development.

We list it in our docs under 'other services'[0], where it is called out for local development.

Because it has an API[1] you can use it for integration testing as well.

Because it runs in docker, you can use it with any stack, not just ruby.

I would not run it in a prod setting.

0: https://fusionauth.io/docs/get-started/download-and-install/...

1: https://mailcatcher.me/#api

A customer of mine runs a service that sends email notifications and OTP codes. We use Mailcatcher to look at the emails without having to wait for them to get to our real inboxes. Furthermore it catches all the emails, even the ones sent to somebody@example.com so we can have totally fake email addresses in our dev databases.