smtp4dev

smtp4dev is a dummy SMTP server for developers, available on Windows, Linux, and Mac OS-X. It captures all outgoing emails from your application during testing, ensuring no messages reach actual customers. The tool supports advanced features like OpenAPI/Swagger API, IMAP/POP3 access for message retrieval, SMTP session logging, and UTF8 support. You can inspect multipart MIME, validate HTML compatibility, and simulate various viewport sizes. It also handles TLS/SSL with auto-generated certificates, authentication, and multiple mailboxes with routing rules. Additionally, you can reply, compose, relay messages automatically, and use scripting expressions for error simulation.