What is a Content Delivery Network (CDN)?
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You send an email with images, and a subscriber opens it in Tokyo while another opens it in Toronto. Both see the images load quickly because the images aren't coming from a single server in one location. They're served from the nearest node in a Content Delivery Network (CDN).
A CDN is a network of servers distributed geographically that cache copies of your content. When you upload images to a CDN-enabled hosting provider, those files get replicated to servers around the world. When a user requests the file, the CDN routes the request to the nearest available server. The result: faster load times and less strain on any single origin server.
For email specifically, CDNs matter because images are loaded at open time, not at send time. A subscriber opening your email on a slow connection in a remote location will experience your image loading from wherever they happen to be. CDN-hosted images load faster in those situations than images served from a single regional server. Faster image loading contributes to a better subscriber experience, and there's evidence that email clients are more likely to display images from fast, reliable domains.
CDNs also add reliability. If one server has a problem, requests automatically route to the next nearest node. That redundancy means your email images won't go blank because a single server had downtime during a busy campaign window. Most major ESPs handle CDN hosting for email images automatically. If you're hosting images yourself, services like Cloudflare offer CDN features at the free tier, and most cloud storage providers like AWS S3 can be paired with a CDN distribution for image hosting. Always serve images over HTTPS, not HTTP, for both security and deliverability reasons.
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