How does the hostname used for image hosting affect email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jul 2025
Updated 19 Aug 2025
7 min read
It's easy to overlook the seemingly minor details in email marketing, but sometimes those details can have a significant impact on your deliverability. One such area that often gets minimal attention is the hostname used for hosting images within your emails. Many marketers assume that as long as an image loads, its hosting location or associated domain doesn't really matter.
However, this isn't always the case. Email service providers (ESPs) and mailbox providers (MBPs) analyze every component of an incoming email, and that includes the domains linked within the message, such as those pointing to your images. While the direct impact might not be as overt as a problematic sending IP or a missing SPF record, the hostname of your image server can subtly influence how your emails are perceived and whether they land in the inbox or the spam folder.
Understanding this nuance is crucial for maintaining a strong sender reputation and ensuring your visually rich emails consistently reach your subscribers. We'll explore why the hostname matters and what you can do to optimize your image hosting for better email deliverability.
The role of hostname in reputation
When we talk about the hostname for image hosting, we're referring to the domain name that appears in the URL of your images within your email. For instance, if your image URL is https://cdn.example.com/images/logo.png, then cdn.example.com is the hostname. This hostname, even though it's not your primary sending domain, still carries its own reputation.
Mailbox providers evaluate all domains present in an email, including those associated with images, to build a comprehensive picture of the sender's trustworthiness. If the hostname used for image hosting has a poor reputation (perhaps due to being used by spammers), it can cast a shadow on your entire email, potentially leading to deferrals or even outright blocking. This is a subtle but important factor in determining email success.
While the primary domain reputation tied to your email's From address is paramount, supporting domains, like those used for image hosting, contribute to the overall trust score. If you're using a third-party content delivery network (CDN) or hosting service, the generic hostname they provide might be shared among many users, some of whom could be engaged in less-than-ideal sending practices.
What is a hostname?
A hostname is the unique label assigned to a device connected to a computer network that is used to identify the device in various forms of electronic communication. In the context of email images, it's the domain or subdomain where your image files are stored and served from. For example, in https://images.yourdomain.com/picture.jpg, images.yourdomain.com is the hostname. This hostname must have a corresponding PTR record for proper lookup, as per common sender guidelines.
Shared hosting versus dedicated subdomains
This is where the choice of image hosting gets interesting. Many businesses use popular CDN services like Cloudfront or AWS S3 for their image assets. While these services are robust and reliable, the default hostnames they provide often reflect the shared nature of their infrastructure (e.g., d1234.cloudfront.net or yourbucket.s3.amazonaws.com). This means your image hostname might be shared with potentially thousands of other users.
When you use a shared hostname, the reputation associated with that hostname is also shared. If other users on the same shared hostname engage in practices that lead to blocklistings (or blacklists) or spam complaints, it could inadvertently affect your email deliverability, even if your own sending practices are pristine. Mailbox providers might apply filters more broadly based on the overall reputation of the shared domain.
This is why using a dedicated subdomain for your image hosting is often recommended. By creating a CNAME record that points a subdomain like img.yourdomain.com to your CDN's hostname, you effectively "brand" your image hosting. This custom subdomain's reputation becomes intrinsically linked to your primary sending domain, giving you more control and shielding you from the negative actions of others on a shared infrastructure. Learn more about how to improve your domain reputation, and how it impacts email deliverability.
Shared CDN hostname
Using the default hostname provided by a CDN or storage service, such as d1234.cloudfront.net or yourbucket.s3.amazonaws.com.
Reputation sharing: Your image host's reputation is influenced by all users of that shared domain, which can be unpredictable.
Less control: You have minimal control over the reputation of the root domain. A shared blocklist (or blacklist) can impact you.
Branding: URLs for images won't reflect your brand, potentially raising slight red flags with discerning filters.
Dedicated custom subdomain
Setting up a CNAME record to point a subdomain of your sending domain (e.g., img.yourcompany.com) to your CDN's hostname.
Reputation isolation: Your image host's reputation is directly tied to your domain, giving you more predictable results.
Improved branding: Image URLs are consistent with your brand, enhancing trust and a professional appearance.
Technical considerations for image hosting
Implementing a custom subdomain for image hosting involves a simple DNS record change. You'll typically create a CNAME record in your domain's DNS settings that points your chosen subdomain (e.g., images.yourdomain.com) to the hostname provided by your image hosting service. This ensures that when a mailbox provider looks up the domain of your images, it resolves back to a subdomain of your primary domain.
Beyond the hostname, it's also important to ensure that your image hosting serves content over HTTPS (SSL/TLS). Most modern CDNs and hosting solutions do this by default, but always confirm. Secure connections build trust with mailbox providers and prevent "mixed content" warnings that can deter recipients. For those interested in deeper technical aspects of image hosting, reviewing documentation on S3 buckets and custom domains can be insightful.
Furthermore, consider the broader impact of your choices. For instance, sometimes images linked from your domain in spam emails can cause deliverability issues, reinforcing the need for tight control over all aspects of your domain's reputation, including image hostnames. This holistic view helps maintain positive sender scores.
Finally, while not strictly about hostnames, ensuring your images are optimized for web use (compressed, appropriate dimensions) can indirectly affect deliverability by reducing email size. Large email sizes can sometimes trigger spam filters or lead to slow loading times, which can negatively impact recipient engagement and, by extension, your sender reputation. How email file size and image hosting affect Gmail deliverability is a pertinent question here.
Example CNAME record for image hostingDNS
images.yourdomain.com. IN CNAME d1234.cloudfront.net.
Best practices for image hosting hostnames
Use a custom subdomain: Set up a CNAME record to brand your image hosting, like img.yourdomain.com.
Secure connections: Always serve images over HTTPS to ensure secure loading and build trust.
Optimize image size: Compress images to reduce email load times and overall email size, minimizing potential spam triggers.
Monitor reputation: Keep an eye on the reputation of your image hosting domain, just as you would your sending domain, especially if you're experiencing email deliverability issues.
Optimizing image hosting for inbox success
While the hostname used for image hosting might seem like a small detail, its impact on your email deliverability and sender reputation can be significant. By taking control of this aspect of your email infrastructure, you reinforce your brand's authenticity and minimize potential risks associated with shared hosting environments. This approach is fundamental to ensuring your emails consistently land in the inbox.
A properly configured image hosting hostname contributes to a cohesive and trustworthy email footprint, aligning all elements of your message under your brand's reputation. This attention to detail can be a differentiator in crowded inboxes and helps bypass stringent spam filters that scrutinize every linked domain.
Prioritizing a dedicated and well-reputed hostname for your images is a key component of a robust email deliverability strategy. It's a proactive step that ensures your visual content supports, rather than hinders, your efforts to connect with your audience.
Views from the trenches
Best practices
Always use a custom subdomain for image hosting, like images.yourdomain.com, for better brand alignment and reputation control.
Ensure your image hosting subdomain has a valid SSL certificate so images are served over HTTPS.
Monitor the deliverability of emails containing images closely, especially if using a new image hosting setup.
Avoid generic or shared third-party hostnames that could carry a poor reputation from other users.
Common pitfalls
Ignoring the reputation of third-party CDNs or image hosting services, which can negatively impact deliverability.
Using very long or unbranded URLs for images, which might look suspicious to spam filters.
Not configuring CNAME records correctly, leading to images not loading or reputation not being properly attributed.
Overlooking image optimization, resulting in large email file sizes that can trigger filters.
Expert tips
Integrate image hosting directly with your primary domain using a CNAME record.
Regularly audit image links in your emails to ensure they resolve correctly and securely.
Maintain consistent branding across all elements of your email, including image hostnames.
Consider the geographic location of your image server if your audience is heavily concentrated in certain regions.
Marketer view
Marketer from Email Geeks says that while Cloudfront and S3 are robust services for hosting images, the key deliverability factor is the specific hostname used for those images, not just the service itself. It's crucial to use a dedicated subdomain rather than a generic one provided by the service.
2024-03-15 - Email Geeks
Expert view
Expert from Email Geeks says that the main thing that will affect your delivery is the hostname used in the URL for your images. If it's a shared hostname, you're sharing content reputation with all other users, which is a factor outside your control. Using a custom subdomain of your own domain gives you more control over that specific reputation.
