What are the deliverability and security implications of embedding versus hosting email signature images?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 18 Aug 2025
7 min read
Email signatures are a crucial part of professional communication, serving as a digital business card that reinforces brand identity and provides contact information. While seemingly simple, the technical implementation of images within these signatures can significantly impact whether your emails land in the inbox or are flagged as suspicious. The core decision often boils down to embedding images directly within the email or hosting them externally and linking to them.
Each method carries distinct implications for both deliverability and security, affecting how your messages are handled by various email clients and spam filters. Understanding these differences is key to maintaining a strong sender reputation and ensuring your communications reach their intended recipients without issues.
My goal is to outline the pros and cons of each approach, helping you make an informed decision that balances visual appeal with optimal email performance and security. We'll explore how these choices influence email deliverability, potential security vulnerabilities, and recipient experience across different platforms.
The two approaches: embedded versus hosted
When we talk about images in email signatures, we generally refer to two main methods: embedding and hosting. Each method dictates how the image data is transmitted and displayed within the email.
Embedded images, sometimes referred to as inline images or CID (Content-ID) images, are encoded directly into the email's HTML code as Base64 strings or attached via MIME. This means the image data is physically part of the email file itself. When a recipient opens the email, the image is immediately available because it's already there, similar to how an attachment works. This method ensures the image displays even if the recipient is offline or has image blocking enabled by default.
Hosted images, on the other hand, are stored on an external web server or a Content Delivery Network (CDN) and linked via a URL in the email's HTML. When the recipient opens the email, their email client fetches the image from the specified URL. This approach keeps the email's file size smaller, as the image data is not included directly in the message. For more details on these approaches, Mail-Signatures provides a comprehensive comparison.
Embedded images (inline)
Data inclusion: Image data is part of the email's raw message.
Offline viewing: Images display even without an internet connection.
Email size: Significantly increases the total size of the email.
Hosted images (linked)
Data inclusion: Image data resides on an external server.
Offline viewing: Images may not display without an internet connection.
Email size: Keeps the email size small and lightweight.
Deliverability implications
The choice between embedding and hosting images has significant email deliverability implications. Large email sizes, often a result of embedded images, can negatively impact how your emails are perceived by mailbox providers.
For embedded images, the primary concern is the increased email file size. Mailbox providers, such as Gmail and Yahoo Mail, may view overly large emails as suspicious or potentially spam. This can lead to your emails being directed to the spam folder, or even rejected outright. Additionally, embedded images can sometimes appear as generic attachments, which can be confusing and unprofessional for recipients. This also means that image-only emails can be particularly problematic.
For hosted images, the email's size remains minimal, which is generally beneficial for deliverability. However, hosted images introduce a dependency on external servers. If the hosting server is slow, unreliable, or becomes unavailable, your images might not load, resulting in broken image icons or a poor recipient experience. Furthermore, many email clients block images by default until the recipient explicitly chooses to display them, impacting the immediate visual impression of your signature.
Security is a paramount concern, especially for organizations that handle sensitive information. Both embedded and hosted images have unique security profiles.
Embedded images, by being self-contained, generally pose fewer risks related to external tracking or malicious server interaction once the email is delivered. However, their larger size can be seen as a potential vector for hiding malicious code or simply overloading systems, which some security protocols might flag. If not properly formatted, embedded images can also trigger security warnings in paranoid systems, as mentioned in the Email Geeks Slack thread.
Hosted images introduce the element of external communication. When an email client fetches an image from an external server, it creates a potential vulnerability if the hosting server is compromised or serves malicious content. This is why using HTTPS/SSL for image hosting is crucial to encrypt the connection and verify the server's identity. Without proper security measures, hosted images could be used for pixel tracking (to confirm email opens), which some privacy-conscious recipients or security systems might view negatively. For a deeper dive, Exclaimer offers guidance on image security.
Regarding the hostname for image hosting, using a dedicated and reputable CDN or S3 buckets with custom domains is generally recommended to avoid associating your main sending domain with image fetching, which can sometimes be seen as less secure by strict filters.
Best practices for email signature images
Considering both deliverability and security, hosted images often present a more favorable option for email signatures, especially for organizations that send a high volume of emails. The key is to implement them correctly.
I recommend always opting for hosted images for several reasons:
Reduced email size: Smaller email file sizes are less likely to be flagged by spam filters and load faster.
Consistency: You can update the image on the server without resending emails.
Centralized control: Easier management of brand assets.
To boost email deliverability rates and maintain good sender reputation, ensure your hosted images are optimized for web use (compressed, appropriate dimensions), and served over HTTPS from a reliable domain.
Optimizing hosted signature images
Image optimization: Compress images to the smallest possible file size without compromising quality. Use appropriate dimensions.
Secure hosting: Host images on a secure server (HTTPS) to prevent security warnings and maintain trust. Consider a CDN.
ALT text: Always include descriptive ALT text for accessibility and in case images don't load.
Testing: Thoroughly test your signature across various email clients and devices to ensure consistent display.
Keeps email size small, generally better for deliverability.
Display reliability
Always displays, even offline, but can appear as attachments in some clients.
Requires internet connection. Can be blocked by default in some Microsoft Outlook clients.
Security risks
Lower external tracking risk. Can trigger warnings if malformed or excessively large.
Potential for malicious content from external host. Requires HTTPS/SSL.
Updates and control
Requires resending emails to update signature images.
Can update images on server without resending. Centralized management.
Views from the trenches
Best practices
Always host email signature images on a reputable server or CDN with HTTPS enabled.
Optimize image file sizes to ensure quick loading and minimal impact on email size.
Use clear and concise ALT text for all signature images for accessibility.
Regularly test your email signatures across various email clients and devices.
Common pitfalls
Embedding large images directly, leading to increased email size and potential spam flagging.
Hosting images on unreliable or unsecure (HTTP) servers.
Neglecting to include ALT text, making signatures inaccessible if images don't load.
Failing to test signatures, resulting in broken displays across different email clients.
Expert tips
Consider using a subdomain specifically for image hosting to better manage sender reputation.
Implement DMARC, SPF, and DKIM to bolster your email authentication and deliverability.
Monitor DMARC reports to identify and troubleshoot issues related to image linking.
Keep signature designs simple and professional to avoid over-reliance on images.
Expert view
Expert from Email Geeks says attached or embedded images, typically via MIME, mean the image is directly part of the message content itself.
2024-06-14 - Email Geeks
Expert view
Expert from Email Geeks says remotely linked images are hosted externally, and while various platforms offer this, it is not strictly required for reputation or deliverability purposes.
2024-06-14 - Email Geeks
Making the right choice for your signatures
The decision between embedding and hosting email signature images involves a trade-off between immediate display reliability and long-term deliverability and security. While embedded images guarantee immediate visibility, they come with the baggage of increased email size and potential flagging as attachments.
Hosted images, when implemented with best practices such as optimization and self-hosting over HTTPS, offer a more robust solution for overall deliverability and a cleaner recipient experience. This approach helps maintain a good domain reputation and mitigates many common issues associated with email images.
Gmail and 
Yahoo Mail, may view overly large emails as suspicious or potentially spam. This can lead to your emails being directed to the spam folder, or even rejected outright. Additionally, embedded images can sometimes appear as generic attachments, which can be confusing and unprofessional for recipients. This also means that image-only emails can be particularly problematic.
Microsoft Outlook clients.
