Does using a different domain for CDN hosted images in emails affect deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 Aug 2025
Updated 16 Aug 2025
9 min read
One of the common questions I hear from senders, especially those managing large email programs with extensive product catalogs, is whether using a Content Delivery Network (CDN) for images on a different domain than their sending domain impacts deliverability. It's a valid concern, given how much focus we place on domain reputation and authentication. After all, you want your emails to land in the inbox, not the spam folder.
The good news is that, for the most part, using a different domain for CDN-hosted images in your emails will not negatively affect your deliverability. Email service providers (ESPs) and mailbox providers (MBPs) primarily evaluate the sending domain for reputation and authentication. The domain hosting your images is generally treated separately.
However, there are nuances and best practices you should be aware of to ensure your images load correctly and don't inadvertently trigger spam filters. The key lies in understanding how mailbox providers view linked content and ensuring that all domains associated with your email, even image domains, maintain a positive standing. Let's dive deeper into why this is the case and what considerations are important.
Image hosting and fundamental deliverability
When an email is received, mailbox providers perform various checks before delivering it to the inbox. These checks primarily focus on the sending domain and IP address, scrutinizing elements like SPF, DKIM, and DMARC records to verify the sender's authenticity. Images, on the other hand, are external resources that are typically downloaded by the recipient's email client only after the email has landed in the inbox, or sometimes after the recipient opens the email.
CDNs (Content Delivery Networks) are designed to serve content quickly and efficiently by distributing it across many servers globally. This is beneficial for emails because it reduces the load time for images, improving the recipient's experience. Using a CDN for images is a common and recommended practice for email marketing because it helps optimize performance and ensures images display correctly, regardless of the recipient's location.
The critical distinction is that the domain hosting your images (the CDN domain) is not the domain sending the email. Therefore, it doesn't directly influence the authentication checks that determine whether your email passes DMARC, SPF, or DKIM. My general finding is that the From: header domain not being present in your links has little to no effect on your email deliverability itself.
However, images can still impact deliverability indirectly. For example, if images are too large or improperly formatted, they can slow down email loading times or even fail to display, leading to a poor user experience. An email that is entirely image-based with little text can also raise red flags with spam filters, as this is a tactic often used by spammers. Ensuring your images adhere to best practices is crucial.
Domain reputation and its influence
While the image hosting domain doesn't directly affect your email's authentication, its own reputation can still play a role. If the CDN domain (or any domain you link to in your email, for that matter) has a poor reputation, it could potentially lead to issues. Mailbox providers might flag emails containing links to known problematic domains, even if those links are just for images.
This is why using reputable CDNs is important. Major CDN providers typically maintain excellent reputations due to the sheer volume and diversity of legitimate content they host. However, if you are self-hosting images on a domain that has been compromised or used for malicious activities, that could be an issue.
I've seen rare cases where a very poor reputation associated with an image domain led to spam delivery issues. This isn't about the domains being different but about the inherent reputation of the image domain itself. If the domain is on a public blacklist (blocklist) or has a history of hosting malware or phishing content, that's a red flag. Thankfully, if you're using a well-known CDN for your legitimate product images, this risk is generally minimal because their core business depends on maintaining a solid reputation.
Best practices for image hosting
Use a CDN: Employ reputable CDNs to host your images for optimal loading times and reliability.
Custom subdomain: If possible, map your CDN to a subdomain of your main sending domain (e.g., images.yourcompany.com). This provides a consistent brand experience and leverages your existing domain reputation. For more details, see our article on S3 buckets and custom domains.
Monitor image domain reputation: Even if using a different domain, ensure it doesn't get listed on blocklists (blacklists). This is less common for image domains but still possible if the CDN is misused by other users.
Use HTTPS: Always serve images over HTTPS to encrypt the connection and enhance trust. This is generally a good security practice and can prevent some warnings from email clients about insecure content. We have a guide on HTTPS and email links for more.
Common scenarios and considerations
While using a separate domain for CDN images is generally fine, it's worth considering common scenarios that come up. Many businesses opt to use a subdomain of their primary marketing or website domain for image hosting. For example, if your sending domain is mail.yourcompany.com, you might use images.yourcompany.com for images. This approach keeps all your associated domains under the same umbrella, reinforcing your brand identity and potentially benefiting from the overarching positive reputation of your main domain.
When you map your CDN to a subdomain, you're essentially telling the world that this subdomain is also part of your brand. If you're using a major CDN like Akamai or Amazon CloudFront, you'll typically set up a CNAME record in your DNS to point your chosen subdomain to the CDN's endpoint. This makes the CDN's infrastructure appear as part of your domain.
The potential pitfall arises if you use a generic, uncustomized CDN domain that you don't control, such as s3.amazonaws.com for Amazon S3 bucket hosting. While these are legitimate services, such domains are shared by countless users. If another user on that shared domain engages in malicious activity, it could, in theory, impact the reputation of the shared domain, which might then indirectly affect your images. This is rare for large, well-managed services but is a theoretical risk. Custom domains (subdomains) mitigate this by associating the image hosting directly with your brand's established reputation.
Domain alignment in practice
When we talk about domain alignment in email, it's typically in the context of email authentication protocols like SPF, DKIM, and DMARC. These protocols ensure that the domain in the From header of your email aligns with the domains used in SPF and DKIM. This alignment is crucial for passing DMARC and proving your email's legitimacy.
Image domains, however, are not subject to these same alignment checks. Mailbox providers don't expect the domain hosting an image to match the email's sending domain for authentication purposes. They treat image links similarly to any other external link within the email content. Therefore, a mismatch between your sending domain and your image CDN domain won't cause DMARC failure or SPF permerror issues.
Where domain discrepancies do matter significantly is with your core sending, tracking, and primary call-to-action links. For instance, if your email's From address uses a different domain from your Reply-To address, that can cause deliverability issues. Similarly, if your tracking domain doesn't align with your sending domain, it can impact your sender reputation. But for images, the rules are much more relaxed. For example, some mail providers like Google are more concerned about your new domain reputation rather than the image CDN domain itself.
My advice is to focus your efforts on ensuring your sending domain is properly authenticated with SPF, DKIM, and DMARC, and that its reputation remains strong. For images, prioritize speed, responsiveness, and using a reliable, reputable CDN, ideally with a custom subdomain. If you're encountering deliverability problems, it's highly unlikely that your separate image CDN domain is the root cause, assuming that domain itself doesn't have a bad reputation.
Domain Type
Example
Deliverability Impact
Sending Domain
mail.yourcompany.com
Crucial for authentication (SPF, DKIM, DMARC) and overall sender reputation.
Image CDN Domain (Custom)
images.yourcompany.com
No direct authentication impact. Positive reputation of this domain is key for image loading. Recommended for brand consistency.
Image CDN Domain (Shared)
s3.amazonaws.com
No direct authentication impact. Small, theoretical risk if the shared domain's reputation is severely damaged by other users.
Always map your CDN to a subdomain that you control, ideally one related to your main brand.
Use HTTPS for all image links to ensure secure loading and avoid mixed content warnings in email clients.
Optimize image sizes and formats to ensure quick loading, enhancing user experience.
Monitor your main sending domain's reputation diligently, as this is the primary factor for deliverability.
Ensure your DNS records (CNAME for CDN mapping) are correctly configured and propagated.
Common pitfalls
Using generic, shared CDN domains that you don't control, which might inherit negative reputation from others.
Not optimizing image sizes, leading to slow load times and potentially frustrating recipients.
Ignoring the reputation of the image hosting domain, even if it's less critical than the sending domain.
Over-reliance on images without sufficient text content, which can trigger spam filters.
Not checking if the CDN domain is on any blocklists (blacklists), although it's rare.
Expert tips
Prioritize the reputation of your sending domain above all else, as it's the main factor for inbox placement.
Consider the user experience; fast-loading, high-quality images contribute to positive engagement signals.
For large volumes of images, a CDN is almost always the right choice for performance and scalability.
If you face deliverability issues, first check your authentication, content, and list hygiene before looking at image domains.
A unified domain strategy (using subdomains for various email functions) simplifies management and brand consistency.
Marketer view
Marketer from Email Geeks says that the image domain should not have any impact on email deliverability.
2023-03-22 - Email Geeks
Marketer view
Marketer from Email Geeks says that the only potential issue would be if you were using an image domain that you did not control, like generic Amazon S3 domains.
2023-03-22 - Email Geeks
Final thoughts on image hosting and deliverability
To summarize, using a different domain for CDN-hosted images in your emails typically does not negatively affect deliverability. The primary factors for inbox placement remain the reputation and proper authentication of your sending domain. While an image domain's own reputation is a minor consideration, it's generally not a direct cause of deliverability issues unless that specific domain has been severely blacklisted (blocklisted) or is known for malicious activity, which is rare for reputable CDN providers.
My recommendation is to focus your efforts on maintaining a strong sender reputation for your email sending domain and ensuring proper SPF, DKIM, and DMARC implementation. For images, use a reliable CDN and, if possible, leverage a subdomain of your primary domain for consistency and to reinforce your brand's overall reputation. This strategy provides both performance benefits and peace of mind regarding deliverability.
Keep an eye on the bigger picture of your email program, from content quality and list hygiene to engagement metrics. These elements will have a far greater impact on your deliverability than the domain used for your CDN-hosted images.
Amazon CloudFront, you'll typically set up a CNAME record in your DNS to point your chosen subdomain to the CDN's endpoint. This makes the CDN's infrastructure appear as part of your domain.
Amazon S3 bucket hosting. While these are legitimate services, such domains are shared by countless users. If another user on that shared domain engages in malicious activity, it could, in theory, impact the reputation of the shared domain, which might then indirectly affect your images. This is rare for large, well-managed services but is a theoretical risk. Custom domains (subdomains) mitigate this by associating the image hosting directly with your brand's established reputation.
Google are more concerned about your new domain reputation rather than the image CDN domain itself.
