Can email sender reputation be harmed by third-party image or link hijacking?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 May 2025
Updated 19 Aug 2025
7 min read
Email sender reputation is a critical factor in deliverability, determining whether your messages land in the inbox or are flagged as spam. It's a complex system influenced by various factors, from your sending volume and bounce rates to how recipients interact with your emails.
A question I've heard recently, and one that rightly sparks concern, is whether a legitimate sender's reputation could be compromised by malicious third parties hijacking their images or links. It's a frightening thought, considering how much effort goes into building and maintaining a strong sender reputation.
The idea of doing everything right and still seeing your deliverability suffer due to external, nefarious actions is a daunting prospect for any email marketer. Let's delve into how this could potentially happen and what measures are in place to mitigate such risks.
What is email sender reputation and why does it matter?
Your email sender reputation is essentially a trust score assigned by internet service providers (ISPs) like Google and Outlook to your sending IP address and domain. A high reputation means your emails are likely to reach the inbox, while a low reputation can lead to messages being sent to spam folders or even rejected entirely. This score is dynamic and based on past sending behavior, as outlined by email reputation systems.
ISPs analyze various signals to determine your reputation. These include your spam complaint rates, bounce rates, the use of spam traps, and engagement metrics such as opens and clicks. They also consider the content of your emails, the authenticity of your sending domain, and whether your IP or domain is listed on any email blacklists (or blocklists).
Maintaining a good sender reputation is foundational for effective email marketing and communication. Without it, your carefully crafted emails might never reach their intended audience, impacting everything from customer engagement to sales and critical operational communications. This is why understanding how email sending practices affect your domain reputation is so important.
How third-party hijacking works
Third-party image or link hijacking occurs when a malicious actor embeds images or links from your domain into their own spam or phishing emails. They do this to exploit your established sender reputation. By using legitimate-looking URLs, they hope to bypass spam filters and trick recipients into opening their harmful messages.
This can take several forms, such as embedding an image hosted on your server, or using your tracking links (e.g., in a newsletter) as zero-length links within their spam. The goal isn't necessarily to redirect users to your site, but to piggyback on your domain's credibility to deliver their illicit content. This is closely related to broken link hijacking but applied to email contexts.
While you might be meticulously adhering to all email deliverability best practices, a third party could be actively misusing your assets. This is especially prevalent if you use shared hosting domains or public image hosting services. Understanding how third-party links affect deliverability is a crucial step in safeguarding your email program.
Legitimate use
Purpose: To display marketing visuals and track user engagement within your legitimate campaigns.
Expectations: Emails originate from your authenticated domains, and links lead to your intended landing pages.
User experience: Recipients interact positively, leading to good sender reputation signals.
Malicious hijacking
Purpose: To exploit your domain's trust to bypass spam filters for phishing or spam campaigns.
Expectations: Emails originate from unauthenticated IPs or domains, potentially with malicious landing pages.
User experience: Recipients mark emails as spam, leading to negative signals for your domain.
The risk of unintended association
When your domain's assets are misused, ISPs may detect patterns of abuse originating from your hosted images or tracked links. While they primarily focus on the sending IP and primary domain, an association with spammy behavior, even if unintended, can lead to your domain (or associated IPs) being listed on a blocklist or having its reputation lowered. This is especially true if you are linking to or from shared resources, as discussed in what happens when your domain is on a blocklist.
The impact of hijacking on your reputation
Yes, your sender reputation can indeed be harmed by third-party image or link hijacking, though the severity and duration of the impact can vary. ISPs are constantly evolving their filtering algorithms to differentiate between legitimate senders and spammers. They analyze not just the sending IP, but also the domain reputation, content, and the reputation of any linked domains or hosted images.
If a spammer uses your hosted image in a widespread spam campaign, ISPs might detect a high volume of spam containing links to your image server. This could lead to your image-hosting domain or even your primary sending domain being flagged, especially if your domain lacks strong authentication. However, modern algorithmic reputation systems are often good at distinguishing between a domain legitimately sending emails and a domain whose content is merely embedded in spam. These systems tend to forget minor, isolated incidents quicker than older, manually maintained blacklists (or blocklists).
While it's a concern, for most legitimate senders, the impact is often short-term and less catastrophic than self-inflicted damage (e.g., sending to unengaged lists or high complaint rates). The systems are designed to look for patterns of abuse. A single, isolated incident of hijacking is unlikely to destroy your reputation, but a sustained attack could warrant closer attention and potentially trigger a block. Understanding how email blacklists work can help contextualize these risks.
Hijacking Type
Potential Impact
Mitigation Strategy
Image Hijacking
Minor, often temporary. Could lead to image URL blocklist if sustained or high volume. Affects image loading, not necessarily full email delivery.
Moderate. Increased spam complaints or blocklisting of link domains, impacting overall sender reputation. Can be more severe if linked to malicious content.
High. Direct impact on domain reputation. Can lead to major blocklisting and deliverability issues. Often involves unauthorized use of your domain in the 'From' address.
Proactive measures are your best defense against third-party hijacking. The core principle is to make it as difficult as possible for malicious actors to exploit your assets and to ensure ISPs can easily verify the legitimacy of your emails. This involves a combination of technical configurations and vigilant monitoring.
Implementing robust email authentication protocols such as SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) is paramount. These protocols verify that emails originating from your domain are authorized, making it much harder for spammers to spoof your sending identity. DMARC, in particular, allows you to instruct receiving mail servers on how to handle emails that fail authentication, giving you control over unauthorized use of your domain.
Beyond authentication, actively monitoring your sender reputation and DMARC reports is crucial. These reports provide invaluable insights into who is sending email using your domain and whether it's passing authentication checks. If you notice a sudden spike in failed authentication or suspicious activity, it could indicate that your domain is being misused. Furthermore, regularly checking for your domain on email blocklists (or blacklists) is a good practice to catch any issues early. You can use a blocklist checker for this.
Key protection strategies
DMARC adoption: Implement a DMARC policy with a quarantine or reject setting. This prevents unauthorized emails from reaching inboxes. Utilize a DMARC record generator to set this up correctly.
Secure hosting for assets: Ensure images and other assets linked in your emails are hosted on secure servers. Be wary of using public or shared shortener services if they can be easily abused.
Regular monitoring: Continuously monitor your sender reputation metrics, DMARC reports, and blocklist status. Early detection is key to minimizing damage.
Educate internal teams: Ensure all teams involved in email sending are aware of deliverability best practices and the risks of unchecked affiliate programs or third-party partnerships.
Views from the trenches
Best practices
Actively use DMARC with a p=quarantine or p=reject policy to prevent unauthorized use of your domain in the 'From' header.
Host your email images on a separate, dedicated subdomain with strong security, ideally with SSL enabled to prevent misuse.
Regularly review your DMARC aggregate reports to identify any third-party sources sending emails on your behalf that you don't recognize.
Implement URL tracking for your links that includes unique, harder-to-spoof identifiers for better monitoring.
Common pitfalls
Relying on generic, free URL shorteners or public image hosting services for critical email assets.
Not implementing DMARC, SPF, or DKIM, making your domain an easy target for spoofing and hijacking attempts.
Ignoring DMARC reports or blocklist alerts, allowing misuse of your domain to go unnoticed for too long.
Having affiliate programs or partner networks with loose email sending guidelines that can lead to reputation damage.
Expert tips
Monitor web reputation services that track malicious URLs and domains to identify if your assets are being misused.
Set up alerts for unusual spikes in email volume or bounces from unexpected sources in your DMARC reports.
Consider rate-limiting or IP restrictions on image hosting to prevent large-scale unauthorized fetching of your assets.
Regularly audit your email sending infrastructure to ensure all third-party vendors adhere to your security standards.
Expert view
Expert from Email Geeks says that while this is a concern, it's not theoretical and can definitely happen.
2019-12-11 - Email Geeks
Marketer view
Marketer from Email Geeks says that even if you're doing everything right, an outside party could potentially harm your sender reputation.
2019-12-11 - Email Geeks
Maintaining a robust email reputation
While the prospect of third-party image or link hijacking harming your email sender reputation is a legitimate concern, it's often not the most significant threat facing email marketers. Modern ISPs employ sophisticated algorithms that can usually differentiate between legitimate sending activity and instances where your assets are being misused. These algorithms are designed to be resilient and to attribute reputation appropriately, even when faced with attempts to exploit your domain.
The greatest threats to your sender reputation typically come from within your own email program: poor list hygiene, low engagement, high spam complaint rates, and lack of proper email authentication. By focusing on strong authentication (SPF, DKIM, DMARC) and continuous monitoring of your email deliverability, you can build a robust defense that protects your sender reputation from both internal missteps and external malicious actors.
Google and 
Outlook to your sending IP address and domain. A high reputation means your emails are likely to reach the inbox, while a low reputation can lead to messages being sent to spam folders or even rejected entirely. This score is dynamic and based on past sending behavior, as outlined by email reputation systems.
