Does hosting images locally on an ESP CMS improve email deliverability compared to external hosting like AWS?
Michael Ko
Co-founder & CEO, Suped
Published 17 Jul 2025
Updated 19 Aug 2025
7 min read
When it comes to email deliverability, every detail matters, including how and where you host your email images. A common question I encounter is whether hosting images directly on your Email Service Provider's (ESP) Content Management System (CMS) offers a significant advantage over using external hosting providers like AWS S3. It's a valid concern, as optimizing every aspect of your email infrastructure can contribute to better inbox placement.
The short answer is, it's more nuanced than a simple yes or no. While some might assume a direct link between the sender's infrastructure and image hosting would inherently be more trustworthy to Mailbox Providers, the reality is that other factors often play a much larger role. The perceived trustworthiness often comes down to the domains and IPs involved, rather than the physical location of the server hosting the image.
The basics of image hosting for email
Images are a crucial part of modern email design, enhancing engagement and brand aesthetics. However, they also introduce potential deliverability challenges if not handled correctly. Large image files can slow down email loading times, leading to a poor user experience, and excessively large emails might even trigger spam filters. Understanding how images in emails affect deliverability is the first step.
When an email client renders an email, it fetches images from their hosted locations. If these locations are unreliable, slow, or associated with suspicious activity, it can negatively impact your email's ability to reach the inbox. Mailbox Providers (ISPs) analyze every element of an email, including image URLs, to assess its legitimacy and potential threat.
A key consideration for image hosting is the use of HTTPS/SSL for email links and images. This encryption helps ensure that the image content has not been tampered with in transit, adding a layer of security that ISPs favor. Regardless of where your images are hosted, ensuring they are served over HTTPS is a fundamental best practice for email deliverability.
Hosting images on an ESP CMS
Many ESPs offer their own CMS or content delivery network (CDN) for hosting images. This integrated approach can provide a seamless workflow, as you often upload images directly within the same platform you use to design and send emails. The appeal lies in its simplicity and the assumption that if your ESP handles the hosting, it must be optimized for deliverability.
While using an ESP's CMS for image hosting is generally a safe and convenient option, it typically doesn't offer a significant, inherent deliverability lift compared to well-configured external hosting. The main benefit is usually operational ease and ensuring images are served from a reputable domain controlled by the ESP. Mailbox providers assess the reputation of the image-hosting domain, regardless of whether it's part of your ESP or an external service.
However, relying solely on your ESP's image hosting can sometimes limit your flexibility in terms of advanced image optimization, global CDN distribution, or custom domain usage for image URLs. For most senders, the ESP's built-in solution is sufficient, provided their CDN is robust and maintains a good reputation.
When ESP CMS hosting is ideal
Simplicity: Integrates seamlessly with your email creation workflow.
Reputation: ESPs typically maintain high-reputation domains for their image hosting.
Maintenance: No need to manage separate hosting accounts or CDN configurations.
Considerations for ESP CMS hosting
Customization: Less control over advanced CDN features or specific domain configurations.
Shared resources: Reputation of shared ESP image domains can be impacted by other users.
External hosting and reputation
External hosting, such as using Amazon S3 or a dedicated CDN, offers greater flexibility and scalability. Many senders opt for this approach to leverage features like global content delivery, fine-grained access control, and cost-effectiveness for large volumes of images. The primary concern with external hosting, however, lies in its potential impact on sender reputation and ultimately, deliverability.
The key differentiator with external hosting isn't the platform itself, but rather whether you use a generic third-party domain (e.g., s3.amazonaws.com) or a custom domain for image hosting. Using your own custom domain (e.g., images.yourdomain.com) for images ensures that the reputation of the image host is directly tied to your own brand, rather than a shared third-party IP or domain that could be used by spammers. This is a common best practice as highlighted by email deliverability best practices.
Another critical point is to avoid URL redirection for images. Some filters may interpret redirects as an attempt to obscure the final destination of the image, potentially leading to deliverability penalties. It is always better to link directly to the image file to ensure transparency and maintain trust with Mailbox Providers.
ESP CMS hosting
Ease of Use: Integrated workflow, minimal setup required.
Shared Reputation: Leverages the ESP's established domain, but also susceptible to shared issues.
Control: Limited direct control over CDN configurations or advanced settings.
External hosting (e.g., AWS S3)
Flexibility: Greater control over CDNs, optimization, and scaling.
Dedicated Reputation: Enables use of your own custom domain, tying reputation directly to your brand.
Management: Requires separate management and configuration.
The paramount importance of domain reputation
Regardless of whether you choose an ESP's CMS or an external service, the most significant factor influencing image deliverability is the reputation of the domain from which the images are served. Mailbox Providers (ISPs like Microsoft and Yahoo) employ sophisticated filters that analyze all URLs within an email, including those pointing to images.
If the domain hosting your images is associated with spam, malware, or other malicious activities, it can lead to your emails being flagged, regardless of how reputable your sending domain is. This is why using a custom domain (or subdomain) for images is often recommended, as it gives you direct control over its reputation.
The common impression that having the sender and image host on the same IP makes it more trustworthy is largely outdated. While IP reputation is critical for sending emails, for image hosting, it's primarily the domain's reputation that ISPs scrutinize. The focus should be on ensuring your image domains are not on any blocklist (or blacklist) and maintain a clean sending history. Mailbox providers, as Vero notes, are good at assessing context.
Aspect
Impact on deliverability
Domain reputation
Crucial. Clean domain history and consistent positive engagement are paramount. Avoid domains used for spam.
HTTPS/SSL
Essential for security and trust signals. Unsecured image links can raise red flags.
Image size & optimization
Large images can slow loading and increase email size, potentially impacting deliverability, especially for Gmail and Outlook.
URL redirection
Avoid it. Can be seen as obfuscation by spam filters.
Views from the trenches
Best practices
Always use a custom domain for image hosting, even with external services like AWS S3 or a CDN.
Ensure all image URLs use HTTPS to encrypt data and build trust with Mailbox Providers.
Optimize image file sizes to prevent slow loading times and oversized emails.
Common pitfalls
Using generic third-party domains (e.g., s3.amazonaws.com) for images.
Not using HTTPS for image hosting, leading to security warnings and potential blocking.
Embedding very large image files directly in emails, which increases email size.
Expert tips
Mailbox Providers are highly sophisticated and assess the full context of an email, including image hosting domains.
The reputation of the image hosting domain is more critical than whether it's an ESP's CMS or an external service.
Even with a custom domain on S3, regularly monitor for potential malware or problematic content issues.
Marketer view
Marketer from Email Geeks says there isn't a clear reason why hosting images at the ESP would provide a meaningful deliverability lift.
2020-06-30 - Email Geeks
Marketer view
Marketer from Email Geeks says the main concern with external hosting like AWS S3 is the reputation risks associated with generic AWS URLs, but using a proxy or custom domain mitigates this.
2020-06-30 - Email Geeks
Strategic image hosting for optimal deliverability
Ultimately, the choice between hosting images locally on an ESP CMS or externally like with AWS S3 doesn't inherently dictate your email deliverability. What truly matters is the reputation of the domain serving your images and adhering to fundamental email best practices.
Prioritize using a custom domain for your images, ensuring all links are HTTPS, and keeping your image files optimized. By focusing on these critical elements, you can ensure that your images enhance your emails without negatively impacting your ability to reach the inbox. It's about smart strategy, not just where the server sits.
AWS S3. It's a valid concern, as optimizing every aspect of your email infrastructure can contribute to better inbox placement.
Amazon S3 or a dedicated CDN, offers greater flexibility and scalability. Many senders opt for this approach to leverage features like global content delivery, fine-grained access control, and cost-effectiveness for large volumes of images. The primary concern with external hosting, however, lies in its potential impact on sender reputation and ultimately, deliverability.
Ease of Use: Integrated workflow, minimal setup required.
Shared Reputation: Leverages the ESP's established domain, but also susceptible to shared issues.
Flexibility: Greater control over CDNs, optimization, and scaling.
Dedicated Reputation: Enables use of your own custom domain, tying reputation directly to your brand.
Microsoft and 
Yahoo) employ sophisticated filters that analyze all URLs within an email, including those pointing to images.
