How do S3 buckets and custom domains for image hosting affect email deliverability and reputation?
Michael Ko
Co-founder & CEO, Suped
Published 23 Jun 2025
Updated 19 Aug 2025
8 min read
When sending emails, especially marketing or transactional ones, images are often a critical component. They enhance visual appeal, convey brand identity, and help deliver the message effectively. However, how these images are hosted can significantly impact whether your email lands in the inbox or the spam folder. Services like Amazon S3 (Simple Storage Service) are popular for hosting web assets, including images, due to their scalability and cost-effectiveness. But when it comes to email, there's more to consider than just storage.
The domain from which your email images are loaded plays a crucial role in how email service providers (ESPs) and spam filters evaluate your messages. If image URLs come from unbranded or questionable domains, it can raise red flags, even if your main sending domain has a good reputation. This is because every linked domain in an email contributes to its overall trustworthiness and the sender’s reputation.
Understanding how to properly configure your S3 buckets and, more importantly, how to use custom domains for image hosting, can make the difference between successful inbox placement and having your emails routed to spam. We will explore the technical considerations and best practices to ensure your images don't inadvertently harm your email deliverability or reputation.
The basics of image hosting and email deliverability
Images are not just visual elements; they are external resources loaded from specific web addresses. When you include an image in an email, the recipient's email client will attempt to retrieve it from the provided URL. Spam filters and ESPs like Google and Yahoo scrutinize all URLs within an email, not just the sending domain. This includes links to images, tracking pixels, and any clickable content.
By default, when you host images in an Amazon S3 bucket, their URLs will look something like https://your-bucket-name.s3.amazonaws.com/image.jpg. While functionally sound for web hosting, using these default Amazon URLs directly in emails can sometimes be problematic. ESPs assess the reputation of every domain linked in an email. A generic domain like s3.amazonaws.com, while belonging to a reputable company, is shared by countless users, including spammers. This can lead to a phenomenon where your emails might be flagged simply because the image host's broader reputation has been tarnished by others. This is a common issue that some users report, such as with Gmail spam filters.
Factor
Impact on deliverability
Domain reputation
If the image hosting domain has a poor or neutral reputation, it can drag down the email's overall score.
Shared vs. dedicated IP/domain
Shared domains can be affected by other users' bad practices.
Content of the image
Spammy or deceptive images can lead to filtering, regardless of hosting.
Accessibility
Broken image links or slow loading times can negatively affect user experience.
The critical role of custom domains
The key to overcoming the challenges of generic S3 URLs lies in using a custom domain. Instead of loading images from your-bucket-name.s3.amazonaws.com, you can configure a CNAME record to point a subdomain of your own, like cdn.yourdomain.com, to your S3 bucket. This way, the image URLs in your emails will appear as https://cdn.yourdomain.com/image.jpg. This branding is crucial because it ensures that all elements of your email, including images, are associated with your established domain reputation, which directly impacts email deliverability.
Using a custom subdomain for image hosting offers several benefits. Firstly, it allows you to build and control the reputation of that specific subdomain. If your main sending domain (yourdomain.com) has a strong reputation, that positive association extends to its subdomains, which can help boost deliverability. Secondly, it presents a more professional and trustworthy appearance to recipients and email filters, reinforcing your brand identity rather than relying on a third-party domain. This also helps with the general issue of hostnames and email deliverability.
Using default S3 URLs
Images are hosted on a generic .s3.amazonaws.com domain. This can lead to reputation issues, as Amazon's shared infrastructure may be abused by others. It can increase the risk of your emails being flagged as spam.
Using custom domains for S3
You set up a CNAME record to point a subdomain of your own (cdn.yourdomain.com) to your S3 bucket. This ensures all image links are branded and reflect your own domain's reputation. It also helps manage external domains within emails.
Mitigating risks: Blocklists and reputation
While using a custom domain is a significant step forward, it doesn't eliminate all potential issues. Your custom domain can still end up on an email blocklist (or blacklist) if its content or usage patterns are deemed suspicious. This could happen if:
Compromised content: If your S3 bucket is misconfigured and open to the public, malicious actors could upload spammy or phishing content to your bucket, which would then be served from your custom domain. This can lead to your domain being added to a blocklist, causing major deliverability issues.
Low-quality image content: Even if not malicious, images with very low resolution, excessive branding, or those often associated with spam (e.g., certain types of emojis or large, unoptimized files) can sometimes trigger filters.
Email content issues: If the emails containing your images are otherwise spammy, even well-hosted images won't save them from the spam folder. Spam filters analyze the entire email, including text, links, and images.
To mitigate these risks, it's essential to follow best practices for S3 bucket security, ensuring that public access is strictly controlled and only necessary permissions are granted. Regularly auditing your S3 bucket policies is crucial. Additionally, ensure that your email content itself adheres to deliverability best practices. Even if your image hosting is perfect, a poorly constructed email can still land in spam, highlighting why fixing deliverability issues is a holistic effort.
Preventing image-related deliverability issues
Always ensure your S3 bucket policies restrict public write access and only allow read access for necessary objects. Regularly check for unintended public exposures. Implementing a custom domain for your image hosting is a crucial step to maintain sender reputation and brand consistency. For more technical setups, consider using AWS security features to further secure your assets.
Advanced strategies for image delivery
For even better performance and security, you might consider using Amazon CloudFront in conjunction with S3. CloudFront is a Content Delivery Network (CDN) service that caches your S3 content at edge locations worldwide. This means images load faster for recipients, regardless of their geographical location. Faster loading times contribute positively to user experience and can indirectly influence deliverability by reducing the likelihood of email clients timing out or flagging slow-loading content. This pairing is often seen as a best practice for hosting email images.
When setting up CloudFront, you can also configure it to use your custom domain, further solidifying your brand's presence and ensuring all assets are served from a trusted, branded source. This also allows for the easy implementation of HTTPS for your image URLs, which is becoming increasingly important for security and deliverability.
Using HTTPS for all image links is now considered a standard practice. Many modern email clients and spam filters prioritize secure content. If your images are served over HTTP, it can trigger warnings or even cause emails to be delivered to spam folders. Ensure your custom domain for image hosting has an SSL certificate configured, which is straightforward with CloudFront.
Example DNS CNAME record for custom domain S3 hosting
Beyond image hosting, maintaining a strong overall domain reputation involves consistent email sending practices, including proper authentication protocols like SPF, DKIM, and DMARC, managing subscriber engagement, and avoiding spam traps. Image hosting is one piece of a larger deliverability puzzle.
Views from the trenches
Best practices
Use custom subdomains for image hosting like cdn.yourdomain.com to brand your assets and leverage your domain's reputation.
Always enable HTTPS for image serving to enhance security and improve deliverability signals for modern email clients.
Consider using a CDN like CloudFront with your S3 bucket to improve image loading speeds and provide global caching.
Regularly review your S3 bucket permissions to ensure they are secure and prevent unauthorized access or content uploads.
Common pitfalls
Relying on default s3.amazonaws.com URLs can expose your emails to broader reputation issues of a shared hosting service.
Forgetting to secure your S3 bucket can lead to subdomain takeover vulnerabilities and damage to your brand reputation.
Not using HTTPS for image links can trigger security warnings in email clients, impacting perceived trustworthiness.
Assuming that good image hosting alone guarantees deliverability without addressing overall email content quality and sending practices.
Expert tips
Set up DNS for your image hosting domain as a CNAME record pointing to your S3 bucket or CloudFront distribution.
Monitor your image hosting domain's reputation using tools like Google Postmaster Tools if applicable for web assets (though less direct than email sending domain reputation).
Implement a consistent naming convention for images stored in S3 to make management and troubleshooting easier.
If using CloudFront, configure appropriate cache behaviors to balance performance and freshness of content.
Marketer view
Marketer from Email Geeks says that image hosting shouldn't inherently cause issues, but more information about spam delivery domains is needed to debug specific problems.
November 29, 2018 - Email Geeks
Expert view
Expert from Email Geeks says that images and any linked assets carry their own reputation, and while large services like Amazon Web Services might have broad reputation, branding links to your own domains is generally recommended.
November 29, 2018 - Email Geeks
Summary of image hosting for deliverability
Hosting email images on S3 buckets, especially with custom domains, can be a powerful strategy for optimizing email deliverability and reputation. By taking control of your image hosting domain's branding and ensuring its security, you send stronger signals of legitimacy to ESPs. While the technical setup requires careful attention, the benefits of improved inbox placement and a consistent brand experience are well worth the effort. Always remember that every element within your email, including where its images are hosted, contributes to your overall sender reputation and your ultimate success in reaching the inbox.
Google and 
Yahoo scrutinize all URLs within an email, not just the sending domain. This includes links to images, tracking pixels, and any clickable content.
Amazon's shared infrastructure may be abused by others. It can increase the risk of your emails being flagged as spam.
