Writes a responsive HTML email from simple content blocks: headings, paragraphs with **bold**, *italic* and [links](https://…), buttons, images, dividers, spacers, two columns and a footer. The markup follows email-client practice — nested tables with role=presentation, inline styles, an Outlook-conditioned fixed-width wrapper, a hidden preheader, bulletproof buttons and a mobile media query that stacks columns — and a plain-text twin is generated for multipart sends. Links must be http(s), mailto: or tel:; images must be absolute http(s) URLs. The preview shows the layout with placeholder images and inert links so nothing is fetched. Everything runs locally.