smtp4dev

smtp4dev is a dummy SMTP server for Windows, Linux, and Mac OS-X that captures all outgoing emails from your development environment, preventing them from reaching real customers. It supports advanced features like OpenAPI/Swagger API, IMAP/POP3 access for message retrieval, SMTP session logging, and UTF8 support. You can inspect multipart MIME messages, validate HTML compatibility, and test TLS/SSL with auto-generated certificates. Additional capabilities include authentication, multiple mailboxes with routing rules, message relay with auto-reply options, and scripting expressions for error simulation—making it an essential tool for comprehensive email testing.