Groundhog

Groundhog is an open-source local email inbox aimed at developers and testers. It intercepts emails sent via SMTP and various provider HTTP APIs (Groundmail, SendGrid, Postmark, Mailgun, Resend, Amazon SES) so that no email leaves the local machine. It features a web UI for viewing emails with live updates, supporting HTML, text, source, headers, attachments, and device previews. Groundhog also supports simulating failure modes and webhooks for testing email workflows. Designed to integrate seamlessly with existing email SDKs by changing base URLs, it helps developers test email sending functionality safely and realistically during development.