How do CDNs serve images securely (HTTPS, caching)?
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Your email's CSS and text load over HTTPS. Then the image loads over HTTP. Some clients will warn recipients about mixed content, or block the image entirely. A Content Delivery Network (CDN) solves this.
But Here's what a CDN does. It stores copies of your images on servers scattered around the world. When your subscriber opens your email in Tokyo, they get the image from a Tokyo server instead of waiting for it to travel from your main server in New York. Fast. Reliable.
HTTPS is non-negotiable. href="/emailalmanac/content/topics/What email client rendering issues should I watch for?" title="Modern email clients (especially Gmail, Outlook, Apple Mail) flag or block images served over HTTP">Modern email clients (especially Gmail, Outlook, Apple Mail) flag or block images served over HTTP. Use a Cloudflare or AWS CloudFront CDN with HTTPS enabled. Your image URLs should start with "https://" always.
Caching keeps things snappy. CDN edge servers cache your images for a set period (usually hours or days). Repeat requests pull from the cache instead of hitting your origin server. You can also use signed URLs to prevent other sites from hotlinking your images, which protects your bandwidth and brand. Test your setup with Review My Emails' SOS hotline if images aren't showing right.
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