smtp4dev

smtp4dev provides a safe testing environment by intercepting emails sent from applications during development, ensuring no messages reach actual recipients. It runs on multiple platforms via .NET Core and offers features like IMAP and POP3 access to view and manage trapped emails, SMTP session logging, and UTF8 support. The tool includes a viewport size switcher for mobile simulation, multipart MIME inspection, and HTML validation reports. With TLS/SSL support, authentication, and customizable mailboxes with rules, you can relay, compose, or auto-reply to messages, plus use scripting to simulate errors for robust testing workflows.