Why are images from a reputable vendor's email blocked by my network?
Michael Ko
Co-founder & CEO, Suped
Published 20 Jul 2025
Updated 17 Aug 2025
7 min read
It can be frustrating when emails from a vendor you trust have images that appear blocked or broken within your network. You might assume it's a simple case of the email client hiding images, but when you try to access the image URL directly and your network still blocks it, the issue is clearly deeper. This scenario points to your internal network's security measures, rather than just basic email client settings.
Understanding why your network would block images from an otherwise reputable sender involves looking into advanced security protocols, content delivery networks (CDNs), and even your vendor's own email infrastructure choices. It is a nuanced problem that goes beyond typical spam filters.
Network and security filter functions
Your network environment is equipped with various security layers designed to protect against a multitude of threats. These layers, which include firewalls, intrusion detection systems, and advanced threat protection (ATP) platforms, meticulously scan incoming traffic, including content linked within emails. When an email with images arrives, these systems don't just check the sender's reputation, they also analyze the URLs where the images are hosted and the content they deliver. This is why a simple client-side image blocking setting isn't the whole story.
Security systems are constantly updated with threat intelligence and can block content based on known malicious patterns or suspicious behaviors. If an image URL or the server hosting it triggers any of these security rules, the image could be blocked. For instance, if an image server has been previously associated with malware distribution or phishing attempts, even if temporarily, your network's defenses might preemptively block access to that resource. This is a common defense mechanism to prevent potential compromises.
Many email clients, like Microsoft Outlook, block external images by default to protect users from web beacons, tracking pixels, and malicious content. However, the scenario you're describing goes beyond these client-side blocks. It suggests a more aggressive network-level intervention. Your internal network's policies and infrastructure play a critical role in determining what content is allowed to load.
Client-side image blocking
This is a user setting in email clients to prevent automatic image loading. It's often for privacy (avoiding tracking pixels) or security (avoiding malicious code embedded in images). Users see an option to Load images.
Network-level image blocking
This occurs when your organization's firewall or security software blocks access to the image's hosting server or URL. It’s a proactive measure against potential threats, regardless of user settings. This indicates a more severe flag against the image source.
CDN reputation and vendor choices
A common practice for reputable email senders, including vendors, is to host their email images on Content Delivery Networks (CDNs). CDNs are designed to deliver content quickly and efficiently by distributing it across multiple servers globally. However, the use of a CDN introduces another layer of complexity. While the vendor itself may have a stellar reputation, the specific CDN or, more precisely, the subdomain on the CDN used for image hosting, might not.
The reputation of the image-hosting domain is distinct from the sending domain's reputation. If other (less reputable) clients on the same shared CDN infrastructure have engaged in malicious activities, that particular CDN IP range or subdomain could end up on a DNS blocklist (DNSBL) or a general email blacklist. Your network's security systems might then block any content from that flagged source, regardless of the reputable vendor using it.
Another factor can be the way vendors configure their links. If image links are not branded with a CNAME to a subdomain of the vendor's main domain, they might appear generic or suspicious. For example, an image link might look like http://randomcdnprovider.com/images/vendor-logo.png instead of http://img.vendor.com/logo.png. Unbranded links can raise red flags for network security solutions, especially in the context of vendor email compromise (VEC) attacks, where attackers impersonate trusted vendors. For more on this, consider resources like Darktrace's insights on supply chain attacks.
This kind of URL, while benign, might be flagged if random-hosting.net has a low reputation or is frequently used by spammers.
Internal policies and vendor configuration
Your organization's network administrators establish specific policies to manage internet traffic and email security. These policies can be highly granular, blocking certain types of content, specific domains, or even entire IP ranges deemed risky. It's possible that the vendor's image hosting domain or IP address, even if generally reputable, might fall under a broad blocking rule within your network's configuration. This is a common situation for companies with very strict security postures.
Sometimes, the issue isn't about reputation but about the vendor's setup choices. If a vendor chooses a low-quality or poorly managed hosting provider for their email assets, this can inadvertently trigger network blocks. While their marketing emails might be legitimate, a security team will prioritize protecting the network from any potential vulnerabilities. This is why vendors in the email space should pay close attention to the infrastructure they use for all email-related content, including images.
It's also worth considering that a vendor might have made a configuration error on their end. A simple misconfiguration, like an incorrectly set CNAME record for image hosting, could lead to network security systems flagging the images. Such issues can sometimes be overlooked by vendors who focus primarily on email content and not enough on the underlying technical infrastructure for hosted assets.
Troubleshooting and prevention
To resolve blocked images from a reputable vendor, the first step is internal investigation. If you can replicate the issue by directly accessing the image URL in a browser on your network, then your IT or security team is the primary point of contact. They can check firewall logs, proxy server settings, and content filtering rules to identify why the specific URL or domain is being blocked. Understanding how images are blocked is key.
If the issue is identified as a network-level block, your IT team might need to whitelist the specific image-hosting domain or IP address. This should be done cautiously, ensuring it aligns with your organization's security posture and doesn't open new vulnerabilities. Communication with the vendor is crucial at this stage. Share the specific image URLs that are being blocked and ask them to investigate their image hosting, CDN, and any associated reputation issues.
For vendors, proactive measures include ensuring all email assets, not just the sending domain, maintain a strong reputation. This means choosing reputable CDNs, branding all image and click-tracking links, and regularly monitoring their associated domains for any blacklist or blocklist appearances. For organizations, regularly reviewing and updating security policies can help balance protection with necessary access to legitimate content. This collaborative approach between sender and recipient is often required to ensure smooth email deliverability.
For your network administrators
Check logs: Review firewall, proxy, and email security appliance logs for specific block reasons. This can pinpoint the exact rule triggered.
Review policies: Assess internal content filtering or domain-blocking policies that might be overly broad.
Consider whitelisting: If the vendor is verified, selectively whitelist the image-hosting domain or URL, if safe.
For the reputable vendor
Check CDN reputation: Monitor the reputation of the CDN or specific subdomain used for hosting email images.
Brand all links: Ensure all image and tracking links are branded to align with the main sending domain.
Review hosting: Verify that image hosting providers are reliable and not associated with known spam or malware activities.
Key takeaways
When images from a reputable vendor's email are blocked by your network, it signals a deeper security or configuration issue than just a simple email client setting. It indicates that your network's sophisticated defense systems have identified something suspicious about the image's source, whether it's the CDN's broader reputation, an unbranded image link, or an overly strict internal security policy.
Addressing this requires collaboration between your internal IT/security team and the vendor. By identifying the exact reason for the block and implementing the necessary adjustments, whether on your network's side or the vendor's, you can ensure that legitimate communications are delivered completely and securely.
Views from the trenches
Best practices
Ensure all email links, including image links, are consistently branded with your domain.
Regularly monitor your CDN and IP reputation to avoid association with problematic senders.
Maintain clear communication channels between marketing and IT teams regarding email infrastructure.
Implement strong DMARC, SPF, and DKIM authentication to bolster sender trustworthiness.
Segment your email sending and image hosting infrastructure to isolate any potential issues.
Common pitfalls
Relying on generic, unbranded CDN links for email images, which can appear suspicious to network firewalls.
Lack of visibility into third-party image hosting reputation or potential blocklist appearances.
Assuming all image blocking is client-side, overlooking network-level security measures.
Failing to review image URLs for any unexpected or redirected domains before sending.
Expert tips
If your network is blocking images, check the URL directly in a browser while on your corporate network. This helps confirm it's a network-level block.
Reach out to your IT or security team with the specific blocked image URLs for log analysis.
Advise your vendors to brand all their image hosting domains with a CNAME record.
Consider asking vendors for their email authentication records and associated domains.
Perform regular email deliverability tests to catch image blocking issues before they impact recipients.
Expert view
Expert from Email Geeks says a firewall or browser policy block is likely, and IT should be consulted. It's important to check if image hosting links use a CNAME to represent a subdomain of the parent domain, as this can lead to extra scrutiny.
2024-09-30 - Email Geeks
Marketer view
Marketer from Email Geeks says it's concerning when a vendor in the email space fails to pass internal firewall restrictions, as such issues are not typically seen with reputable vendors and raise doubts about their practices.
