What are the three approaches to responsive email design — fluid, hybrid, and responsive?

Fluid (Liquid): Uses percentage-based widths so the email scales proportionally on any screen. No media queries required. Limitation: text can become too small or too wide without min/max-width constraints.
Hybrid (Spongy): Combines fluid percentage widths with max-width constraints using MSO conditional comments to fix Outlook rendering. The inner table uses width:100% and max-width in CSS, while Outlook sees a fixed-pixel table via VML/conditional comments. This is the most widely compatible approach today.
Responsive: Uses CSS media queries to change layout at breakpoints (e.g., stack columns below 600px). Works great in clients that support media queries (modern Gmail, Apple Mail, iOS Mail). Outlook desktop clients largely ignore media queries, so a hybrid fallback is still needed.
Outlook VML: Microsoft Outlook uses VML (Vector Markup Language) for background images and gradient fills — standard CSS background-image is ignored.