Does using a different domain for CDN hosted images in emails affect deliverability?

Key findings

• No direct impact: The use of a separate domain for CDN-hosted images is not inherently detrimental to email deliverability.
• Reputation is key: The critical factor is the reputation of the image-hosting domain. A poor reputation on this domain can lead to deliverability problems.
• Domain control: Using domains that are not controlled by your company, such as generic shared CDN domains (e.g., s3.amazonaws.com), may pose higher risks due to their shared reputation.
• Image relevance: Images themselves do not reduce deliverability if properly used, even if multiple are present in an email.

Key considerations

• Check image domain reputation: Regularly monitor the reputation of your image-hosting domain. A clean, established domain is preferred.
• Consider domain alignment (optional): While not strictly necessary for deliverability, some companies prefer to map their CDN to a subdomain of their primary sending domain (e.g., images.yourcompany.com) for branding or perceived consistency.
• Understand general domain reputation: It is important to understand how multiple or external domains affect sender reputation and deliverability. This includes your image hosting domain.
• Avoid known bad domains: Ensure that the CDN domain is not on any email blacklists or blocklists. A compromised domain, even for images, can lead to deliverability issues.
• Link reputation vs. image reputation: While different, both links and image domains contribute to the overall impression of your email. Ensure all domains used in your email (including image hosting hostnames) have good standing. For more on general image considerations, review Kickbox's Email Images & The Inbox guide.

Key opinions

• No deliverability impact: Most marketers believe the image domain itself has little to no direct impact on email deliverability, provided it's legitimate.
• Reputation dependence: The only scenario where an image domain could pose a problem is if it has a very poor or blocked reputation.
• Control over domain: It's important to use a domain you control for image hosting, rather than a generic public CDN URL that others can abuse.
• Common practice: Many marketers map their CDN to a subdomain of their own company's domain for organizational purposes, even if it's not a deliverability requirement.
• Focus on image practices: Marketers emphasize focusing on best practices for images in emails as content elements, rather than the hosting domain specifically, unless the domain itself is problematic.

Key considerations

• Assess current deliverability: If you're already facing deliverability issues, investigate other common causes before attributing them to image hosting domains.
• CDN domain reputation: Confirm that your CDN domain, especially if it's a primary content source for your website, has a strong and reliable reputation.
• Alternative hosting: Unless there's a clear deliverability problem, uploading a large volume of images directly to an ESP's content system might be an unnecessary effort. You may also want to investigate how self-hosting email images affects deliverability as an alternative.
• Email content best practices: Ensure that your email content adheres to general best practices to maximize inbox placement, regardless of image hosting. Vero provides a useful guide on email image deliverability and experience.

Key opinions

• Image domain reputation counts: The reputation associated with the image domain itself can indeed affect spam delivery.
• No impact if reputation is good: There should be no negative impact from simply using images hosted at a different domain, as long as that domain does not have reputation problems.
• Bad image domain equals spam: If an image domain has a particularly bad reputation, it can directly lead to emails being sent to spam folders.
• Avoid uncontrolled domains: Experts advise against using domains that are not under your direct control, as their reputation can be unpredictable and shared.
• Holistic domain reputation: An understanding of your email domain reputation is crucial for all domains used in your email, including image hosts.

Key considerations

• Proactive reputation management: It is better to take preventive measures and ensure all domains linked in your emails have a good standing.
• Monitor blocklists/blacklists: Regularly check if your image domain appears on any email blocklists or blacklists, as this would be a direct red flag.
• General deliverability health: If emails are going to spam, investigate all potential causes, including sending domain reputation, content, and authentication, not just image domains.
• Consistent good practices: Email deliverability is built on consistent good sending practices across all elements of an email. Spam Resource provides insights into general sender reputation factors.

Key findings

• CDN domain usage: CDN domains are specifically designed and commonly used for serving assets like images in emails.
• DNS configuration: Proper CNAME record setup is essential for CDN functionality, ensuring no conflicts with other DNS records.
• Image impact on experience: Issues with email images can disrupt user experience and reduce overall deliverability.
• Best practices for images: Adhering to best practices for image usage is critical to amplify messaging and avoid deliverability issues.
• HTTPS/SSL benefits: Using HTTPS/SSL for email links and images is recommended for improved security and potentially better deliverability.

Key considerations

• CDN setup for email: Ensure your CDN is properly configured for email content delivery, considering any specific requirements or recommendations from your ESP or CDN provider. Bloomreach's Custom Domain Management documentation provides context on CDN domain usage.
• Performance and reliability: While deliverability is key, ensure your CDN provides reliable and fast image loading, as this impacts user experience.
• S3 vs. CloudFront: Consider the advantages and disadvantages of different hosting solutions, such as Amazon S3 versus CloudFront, and their impact on deliverability.
• Consistent user experience: Ensure that email images load correctly across various email clients to maintain a positive recipient experience, as image issues can reduce deliverability.