MailHog

MailHog catches all outgoing email from your development environment and displays it in a web UI instead of actually delivering it. Point your app's SMTP at localhost:1025, and every email lands in MailHog instead of going anywhere. It's a fake mailbox for development. MIT license, Go. Single binary, zero configuration. Start it, configure your app's SMTP to point at it, done. The web UI at port 8025 shows every email with headers, body, HTML rendering, and attachments. Has a JSON API for automated testing: verify emails were sent in your CI pipeline.

Fully free. No paid tier, nothing to pay for. It's a development tool, not a production service.

Every developer who builds email-sending features: use this. Solo to enterprise, the setup is identical: `docker run mailhog/mailhog` or download the binary. Takes 30 seconds.

The catch: MailHog is essentially unmaintained. The last commit was years ago. It works fine for basic SMTP testing, but don't expect updates or new features. For a maintained alternative, Mailpit is a drop-in replacement that's actively developed. Also, MailHog is development-only. Never expose it to the internet, there's no authentication on the web UI.