MJML (Mailjet Markup Language) is an open-source markup language and framework developed by Mailjet to simplify the process of coding responsive emails. By providing a semantic syntax and a rich library of components, MJML enables developers and marketers to create responsive email designs that render consistently across various email clients and devices.

The framework abstracts the complexities of traditional HTML email coding by offering high-level components such as `<mj-section>`, `<mj-column>`, and `<mj-text>`. These components allow users to focus on content creation rather than intricate styling details. MJML's engine then transpiles this markup into responsive HTML, ensuring compatibility even with notoriously challenging email clients like Outlook.

MJML offers multiple tools to accommodate different workflows, including an online editor for immediate use, a desktop application for offline editing, and plugins for popular code editors like Visual Studio Code and Sublime Text. This flexibility allows users to integrate MJML seamlessly into their existing development environments.