How does the hostname used for image hosting affect email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jul 2025
Updated 14 Aug 2025
11 min read
Summary
The hostname used for image hosting can subtly influence email deliverability, primarily through its impact on sender reputation and how mailbox providers perceive your email content. While the direct hosting service (e.g., a CDN like CloudFront versus storage like S3) might seem less critical, the specific hostname used in the image URL is paramount. A generic or shared hostname can dilute your control over reputation, whereas a dedicated or custom subdomain allows you to build and maintain a distinct positive reputation for your image assets. This aligns the image hosting with your primary email sending domain, contributing to a more cohesive and trustworthy sender identity. Ensuring your image hosting setup is well-configured and aligns with your email sending practices is a key part of maintaining good inbox placement. Consider also how S3 buckets and custom domains affect deliverability.
Key findings
Hostname matters: The hostname in your image URLs is more impactful on deliverability than the specific hosting service (CDN vs. cloud storage).
Shared reputation risk: Using generic or shared hostnames, such as a default CDN domain, means your image reputation is shared with other users, potentially exposing you to their negative sending habits.
Underlying service: While CloudFront is a CDN built on S3, their distinct functions mean they are not directly comparable services for email image hosting, though both can be used for it.
Consistent branding: Use a hostname that clearly reflects your brand, ideally a subdomain of your main sending domain, to reinforce trust and authenticity with mailbox providers.
CDN vs. S3: While CloudFront (CDN) is optimized for delivery speed, S3 is primarily for storage. Both can host images, but for email, ensure you're using a setup that allows for custom hostname control.
DNS configuration: Properly configure DNS records (e.g., CNAME) for your custom image hosting subdomain to ensure it resolves correctly and consistently. This is a critical factor for boosting email deliverability rates.
What email marketers say
Email marketers often approach image hosting with practical concerns about deliverability, focusing on the perceived impact of different services and the associated hostnames. While many recognize the importance of images for engagement, the technical nuances of how hostnames affect inbox placement can sometimes lead to confusion. Discussions frequently revolve around choosing between widely used services like Amazon CloudFront or S3, and understanding how these choices translate into actual deliverability outcomes. Marketers tend to prioritize solutions that are reliable, scalable, and, most importantly, do not inadvertently trigger spam filters. The consensus leans towards controlling the hostname as much as possible to maintain a consistent sender identity.
Key opinions
Confusion between services: There's often a lack of clarity regarding the differences between content delivery networks (CDNs) like CloudFront and storage services like S3, and how each might theoretically affect deliverability.
Perceived deliverability issues: Some marketers believe that their current image hosting setup, even with reputable providers, could be contributing to deliverability problems.
Focus on hostname: Many marketers, once informed, pivot their concern to the specific hostname visible in the image URL rather than just the hosting platform itself. This is a core aspect of understanding your email domain reputation.
Shared reputation concerns: There's an awareness that using a generic hostname (e.g., cloudfront.net) means sharing reputation with a vast number of other users, which can be detrimental if those users engage in poor practices.
Key considerations
Cloudflare terms of use: Be cautious with services like Cloudflare for primary image hosting, especially on free or lower-tier plans, as their terms of use may prohibit disproportionate image hosting relative to HTML content.
Branded hostnames: Marketers should aim to use custom, branded subdomains for image hosting, even if using a third-party service, to better control and build their own image reputation.
Image optimization: Regardless of hosting, optimizing image file sizes and formats remains crucial to reduce email size and improve loading times, which indirectly impacts deliverability and user experience. Learn more about how email image issues affect deliverability.
Hosting location relevance: While not directly impacting deliverability as much as hostname reputation, the location of image hosting can affect loading speeds for recipients in different geographies, influencing engagement metrics.
Marketer view
Marketer from Email Geeks indicates confusion arose from being told that CloudFront, an Amazon service, might cause deliverability issues. They asked how using S3, another Amazon service, would mitigate this, suggesting a need for clearer guidance on Amazon's offerings.
26 Feb 2020 - Email Geeks
Marketer view
Marketer from Email Geeks states they initially confused CloudFront with Cloudflare and apologized for the misunderstanding. They added that Cloudflare protects inappropriate content and doxes reporters, advising others to avoid them due to these ethical concerns.
26 Feb 2020 - Email Geeks
What the experts say
Experts in email deliverability emphasize that while the specific platform for image hosting (like CloudFront or S3) has its own technical merits, the critical factor for email deliverability is the hostname itself. The hostname directly contributes to the email's overall trustworthiness and sender reputation, influencing how mailbox providers (ISPs) assess the email. Using a dedicated, branded subdomain for image hosting allows for better control over reputation, isolating it from potential issues arising from shared hostnames. This nuanced understanding is key to optimizing email performance and ensuring that email content, including images, reaches the intended inbox.
Key opinions
Hostname centrality: The hostname within the image URL is identified as the primary factor affecting email delivery, more so than the specific image hosting service.
Reputation sharing: Experts warn that using generic, shared hostnames (e.g., cloudfront.com) for images means sharing content reputation with all other users, which is beyond an individual sender's control. This can lead to being affected by others' poor practices, making it a reason your emails fail.
Subdomain benefits: Utilizing a custom subdomain (e.g., yoursubdomain.cloudfront.com or img.yourdomain.com) for images significantly reduces the degree of shared reputation, linking the image's reputation more directly to the sender's own domain.
Distinguishing services: CloudFront is correctly identified as a CDN, distinct from S3's role as simple storage, even though CloudFront is built on S3. These are not directly comparable services when discussing their primary functions.
Key considerations
Custom domain usage: Always aim to use a custom domain or a subdomain of your primary sending domain for image hosting. This provides more control over your image assets' reputation, similar to how subdomains for outbound email delivery can improve deliverability.
Domain reputation management: The domain used for image hosting should be part of your overall domain reputation strategy. A clean image hosting domain contributes positively to your main sending domain's standing.
Monitoring: Regularly monitor the reputation of your image hosting domain, just as you would your email sending domain, for any signs of blacklisting or poor standing. This ties into the general advice for how email file size and image hosting affect Gmail deliverability.
Expert view
Expert from Email Geeks clarified that CloudFront is indeed an Amazon service, not Cloudflare, and is a CDN built on top of S3. They emphasize that these are distinct services, though related within the Amazon ecosystem.
26 Feb 2020 - Email Geeks
Expert view
Expert from Email Geeks states that the main factor influencing email delivery is the hostname used in the image URL. This highlights the importance of domain reputation associated with image assets.
26 Feb 2020 - Email Geeks
What the documentation says
Official documentation and technical guides from various sources highlight the technical aspects of image hosting and their potential, albeit indirect, impact on email deliverability. They often focus on the broader context of email authentication, domain configuration, and content optimization, within which image hosting hostnames play a role. Key areas of focus include how hostnames are exposed, the implications of shared versus dedicated domains for reputation, and general best practices for integrating images into emails without triggering spam filters. The emphasis is on maintaining control and consistency across all elements of your email sending infrastructure to build a strong sender identity.
Key findings
Hostname visibility: Server hostnames, including those for image hosting, can be visible in email headers. This exposure allows mail servers to assess the reputation of the hostname used.
Impact on deliverability: The configuration of your hostname can indeed influence email deliverability, with poorly designed or configured hostnames potentially leading to increased spam flagging or outright blocking.
URL structure matters: Every image in an email is linked to where it's hosted. The specific URL, including the hostname, dictates how the image is downloaded and displayed. Long URLs could potentially add to email weight, as noted by Kickbox.
Image size effects: Large or uncompressed images increase email size, which can slow loading times and potentially trigger spam filters, impacting deliverability whether they are embedded or hosted externally. Twilio mentions embedding images can affect deliverability if not handled correctly.
Key considerations
Specify authorized hostnames: Documentation (e.g., ScalaHosting) emphasizes specifying which IP addresses and hostnames are authorized to use your domain for sending emails, extending to image hosting.
DNS configuration for images: Proper DNS configuration, including setting up CNAME records for custom image hosting subdomains, is essential for authenticating image sources and ensuring proper resolution.
Optimization for size: Consistently optimize images by resizing and compressing them to minimize email file size. This practice helps avoid issues with spam filters that may flag overly large emails.
Differentiating hosted vs. embedded: Understand the difference between images stored within the email (embedded) and those stored on a web server (hosted). Hosted images rely on the external hostname for deliverability and loading, while embedded images contribute directly to email file size. Gimmio explains the distinction between hosted and embedded images.
Technical article
Documentation from Certa Hosting Blog states that hostnames can indeed impact email deliverability. They clarify that the specific configuration of your hostname plays a direct role in whether your emails are successfully delivered or encounter issues.
22 Mar 2025 - Certa Hosting Blog
Technical article
Documentation from Twilio highlights that embedding images can affect deliverability if not managed properly. They point out that large images can slow down email loading times and that image-only emails may face deliverability challenges.
