Why newsletter creative specs are different from web specs
Newsletter advertising runs in HTML email environments, not browsers. This distinction has profound implications for creative design, technical specification, and compatibility testing. The same creative that renders beautifully in Chrome will often break in Outlook, display incorrectly in Gmail's mobile app, or show nothing at all in a client that blocks remote images by default.
Standard dimensions
Newsletter creatives should be designed for 600px container width — the de facto standard for HTML email. This renders at full width on desktop email clients and scales proportionally on mobile. Wider designs get clipped in Gmail and Outlook. Key dimensions by format:
- Header banner: 600 × 100px to 600 × 200px. Use for brand awareness, simple visual messaging.
- Mid-article banner: 600 × 200px to 600 × 300px. Best for offers with a clear call-to-action button.
- Native text ad: No fixed dimensions — structured text with headline, body, and link. Renders in all email environments including plain-text mode.
- Logo + text hybrid: 600px wide, 80px logo column + copy column. Good for brand sponsorships.
Image optimisation
Email images must be hosted at a stable URL (not in the email body). Optimise aggressively: aim for under 100KB per image. Use JPG for photographs, PNG for graphics with transparency. Include descriptive alt text for every image — Apple Mail Privacy Protection and many corporate email clients block images by default, and alt text is often the only content the reader sees.
HTML and CSS compatibility
HTML email CSS support is dramatically more limited than browser CSS. Supported properties vary by client, but safe bets include: inline styles (always use inline, not stylesheet), table-based layouts (floats and flexbox break in Outlook), max-width, padding, margin, and font-family with web-safe fallbacks. Avoid: CSS grid, position: absolute, background-image in most cases, web fonts (render as fallback in most clients), and JavaScript (ignored everywhere).
Testing before launch
Test every creative in Gmail (desktop and mobile), Apple Mail (macOS and iOS), Outlook 2019/365, and Samsung Mail. These four clients cover approximately 85% of email opens globally. MailAdx's creative review process includes an automated rendering test across these environments and flags any compatibility issues before the creative goes live.
Conclusion
Creative quality is a significant driver of newsletter ad performance. Advertisers who invest in well-specified, thoroughly tested creatives consistently outperform those who repurpose web display assets. MailAdx's creative library includes format templates for each placement type to help advertisers get started quickly.