MailCatcher runs a super simple SMTP server

This is a development tool, not production. For it's intended use it is mature. There has been Git activity within the last year so I am not sure why that would be considered unmaintained by some.

If something is important to someone in particular, they should implement a pull request or see the author's website about making a donation or paying for the development of a particular capability.

Who says it's not for production?

Just because it's not actively developed doesn't mean it's always a development tool.

Looks bug free and feature complete to which I would conclude is more production worthy than most.