I built a simple fake SMTP server that logs received emails and packaged it as a docker container. This can be handy in development, typically as part of a docker-compose.yml of some kind, if the service you’re working on requires SMTP and doesn’t provide an easy escape hatch (like Rails’ ActionMailer interceptors or Django’s console email backend). The SMTP RFC is vast; at the moment, smtp-logger only supports the tiniest bits necessary to make my basic use cases work. But hopefully it’s useful to someone else and easily extended.