MailHog

MailHog is an email testing tool for developers, providing a local SMTP server that captures outgoing emails for inspection and testing without sending them to real recipients. Written in Go by Ian Kent, MailHog solves a common development problem: testing email functionality (registration confirmations, password resets, notifications, newsletters) without risking accidental delivery to real users or requiring external SMTP services. The tool runs as a single binary with no external dependencies, listening on an SMTP port (default 1025) and providing a web-based UI on port 8025 where developers can view captured emails in real-time. Features include: HTML and plain text email rendering with proper MIME multipart support, raw email source viewing for debugging SMTP headers and encoding, attachment viewing and download, real-time email notification via WebSocket, email search and filtering, SMTP authentication support (PLAIN, LOGIN, CRAM-MD5), configurable message storage (in-memory for ephemeral runs, MongoDB or file-based for persistence), sendmail compatibility mode for drop-in replacement of system sendmail, and a RESTful JSON API for programmatic email verification in test suites (enabling integration with Selenium, Cypress, Playwright, and other E2E testing frameworks). MailHog is widely used in Docker Compose setups for local development environments and CI/CD pipelines. As of 2026, it has over 14,000 stars.