smtp4dev

smtp4dev provides a safe environment for testing email functionalities in applications without risking spam to real addresses. It runs on multiple platforms via .NET Core and includes features like OpenAPI/Swagger API, IMAP and POP3 access to manage messages, and detailed SMTP session logging. Developers can inspect MIME parts, check HTML validity, and switch viewport sizes for mobile simulation. The server supports TLS/SSL encryption, authentication, and multiple mailboxes with customizable rules. It also allows message relaying, replying, composing, and scripting for error testing, making it a comprehensive tool for email development and debugging.