Papercut SMTP is a desktop email server and viewer for development that intercepts outgoing mail from your application without delivering it anywhere.
The tool solves the problem of testing email functionality during development by running a lightweight SMTP server on localhost that captures all messages sent to it. When your application points to localhost:25, every email is caught and displayed instantly in a rich interface showing the body, HTML, headers, attachments, and raw message content. Nothing is delivered to real recipients, making it safe for development and testing workflows.
Developers should choose this tool if they need to inspect emails their application generates during development or testing. It suits any project that sends mail and requires visibility into what was sent without the complexity of configuring a real mail server or the risk of accidentally sending test emails to production addresses. The tool runs as a Windows desktop application with installers for 64-bit, 32-bit, and ARM64 architectures, and is also available via package manager or as a Docker container with a web UI. An optional Model Context Protocol server allows AI agents to programmatically inspect messages, assert on content, verify attachments, and clean up between test runs.
The project maintains active development with regular updates to its desktop application and service components. Documentation is comprehensive and covers installation, configuration for multiple programming languages, deployment options including Docker, and TLS authentication setup. The codebase is written in C# and includes both a WPF desktop interface and a web-based UI for the service variant.
