smtp4dev

smtp4dev is a fake SMTP server designed for development and testing environments. It captures email messages sent by applications via SMTP connections and displays them in a local interface, preventing delivery to real recipients during the software development and debugging process.

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• SMTP message interception. The program acts as a local SMTP server, receiving all messages that applications under development attempt to send via the SMTP protocol. Messages are stored locally and are not forwarded to real mail servers.
• Message storage and display. All intercepted emails are shown in a list within the interface. For each message, data such as the sender, recipients, subject, and reception date are displayed.
• Detailed content inspection. Allows examining the full content of each message, including technical SMTP headers, plain text and HTML message body, and encoded attachments.
• HTML message preview. Shows a rendered representation of the HTML content of emails, similar to how an email client would, to verify layout and links.
• Attachment management. Extracts and saves file attachments from email messages to the local file system. Allows opening attachments directly with the operating system's default applications.
• Server parameter configuration. Allows defining the SMTP listening port, interface network address, and server domain name. Supports configuring multiple ports simultaneously.
• TLS/SSL support. Offers the ability to configure secure SMTP connections via TLS/SSL, including specifying X.509 certificates for communication encryption.
• Simulated SMTP authentication. Supports and logs SMTP authentication attempts (such as LOGIN, PLAIN) without requiring valid credentials, allowing testing of authentication flows in applications.
• System notifications. Displays visual alerts or operating system notifications when new messages arrive, with options to customize the behavior of these notifications.
• Message limits and rotation. Configures a maximum number of messages to retain in the internal database. Older messages are automatically deleted when this limit is exceeded.
• REST API. Includes an HTTP REST Application Programming Interface (API) to query, retrieve, and manage messages via programmatic calls, facilitating integration into automated test flows.
• Web user interface. Provides a graphical interface accessible through a web browser, in addition to the desktop application, to manage and review captured messages.
• Headless mode. Can run as a service or background process without needing a user interface, ideal for continuous integration servers or headless environments.
• Data persistence. Stores captured messages in an internal database (typically SQLite), retaining them between application restarts to allow long-term analysis during development cycles.
• Filtering and search. Includes functions to search and filter the list of captured messages by criteria such as sender, recipient, subject, or keywords in the content.

The development of smtp4dev began in 2008 by Richard Wood as a tool to solve problems in the software development process involving the sending of emails. The program is primarily written in C# for the .NET platform. Over the years, it has gone through several iterations and rewrites, including a migration to .NET Core (now .NET) to enable its cross-platform execution on Windows, Linux, and macOS.

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