Why is Microsoft Defender blocking emails on CTA clicks even if domains are not on DNS blacklists?
Michael Ko
Co-founder & CEO, Suped
Published 9 Jul 2025
Updated 18 Aug 2025
8 min read
It can be incredibly frustrating when emails get blocked, especially when you've done your due diligence and confirmed your domains aren't listed on any DNS blacklists or blocklists. A common scenario I encounter is when Microsoft Defender blocks emails specifically at the point of a call-to-action (CTA) click. This issue often points to something beyond traditional IP or domain blocklists.
When users click a link in your email, Microsoft Defender for Office 365 (MDO) performs a real-time scan of the URL. This is part of its Safe Links feature, which is designed to protect users from malicious links. The block isn't occurring because your sending domain is on a blacklist, but rather due to a perceived threat or suspicious activity associated with the linked URL itself.
This typically happens if the destination URL, or any redirect in the chain, is flagged for phishing, malware, or other security concerns. Even if the primary domain of your website is clean, a specific subdomain used for click tracking by your Email Service Provider (ESP) could be the culprit.
This situation can arise from various factors, including the shared nature of some click-tracking domains, suspicious redirects, or even issues with the content on the landing page. It is essential to understand that DNS blocklists primarily target IP addresses or mail domains used for sending spam, not necessarily the integrity of links within the email body at the point of click.
Microsoft Defender (formerly Office 365 Advanced Threat Protection) employs a robust set of technologies to protect users. Unlike traditional spam filters that might solely rely on IP or domain blacklists, MDO focuses heavily on real-time threat intelligence and heuristic analysis.
Its Safe Links feature is designed to rewrite URLs in emails and then scan them at the time of click. This means that even if a link was safe when the email was sent, if the destination becomes malicious later, MDO can still block access. This dynamic protection is a key differentiator. You can learn more about how Microsoft scans links in detail.
Another crucial aspect is URL reputation. Microsoft maintains its own internal reputation scores for URLs. If a URL is associated with phishing, malware distribution, or other suspicious activities, it will be flagged, regardless of the sender's domain or IP address reputation. This is why even a legitimate domain can have a problematic URL if it's been compromised or used in a way that triggers MDO's detection mechanisms.
I often see issues stemming from the use of shared click tracking domains provided by ESPs. If one customer on that shared domain sends out malicious links or gets compromised, it can affect all other users who share that tracking domain. Microsoft's systems proactively scan and sometimes click these links to evaluate the destination, which can lead to blocks if any suspicious activity is detected.
The distinction between DNS blocklists and URL reputation
The distinction between a domain on a DNS blocklist and a URL blocked by Safe Links is critical. A DNS blocklist (or blacklist) primarily identifies IP addresses or domains that have been observed sending spam or engaging in malicious mail activities. These lists are consulted before an email even reaches the recipient's inbox. If your IP or domain is on one of these, the email will likely be rejected outright.
However, with a Safe Links block, the email itself was delivered. The issue arises when the recipient attempts to interact with the link. This implies that the initial email passed various authentication and content checks, but the linked content or redirect chain failed a real-time security assessment by Microsoft (or other security solutions like Barracuda or Mimecast). This is a very different problem than being listed on a blocklist.
Here's a breakdown of the differences I often explain:
DNS blocklist (or blacklist) block
Cause: Your sending IP or domain is flagged for spam or malicious activity by a public or private DNS blocklist.
Detection: Occurs at the email server level, before delivery to the inbox.
Symptom: Emails are rejected with a bounce-back message (NDR), or simply disappear.
Cause: The destination URL, or any redirect in the chain, is flagged as malicious by Microsoft Defender's real-time scanner.
Detection: Occurs when the user clicks the link in the email.
Symptom: Email is delivered, but clicking the link leads to a block page warning about a malicious site.
Resolution: Investigate the linked URL, address any compromises, or adjust redirect chains. Contact your ESP if it's a shared tracking domain.
This clear distinction helps in troubleshooting. If you're getting no bounce messages but experiencing blocks on clicks, your focus should shift from DNS blacklists to the content and redirects of the URLs themselves. This is why email deliverability is such a complex area.
Troubleshooting and resolution strategies
When facing this issue, my first recommendation is to investigate the exact URL that's being blocked. It's not enough to just check your main domain. You need to look at the full click-tracking URL as it appears in the email source, and then follow any redirects manually to see the final destination.
If you're using an ESP, they typically handle click tracking using their own domains or subdomains. If these are shared among many clients, one bad actor could inadvertently impact your campaigns. In such cases, contact your ESP and ask them to investigate the reputation of their click tracking domains. They might need to move you to a different tracking domain or take steps to clean up the shared one.
Sometimes, the issue isn't the tracking domain itself, but what it points to. Ensure your landing pages are secure and free of any suspicious content. If your website has been compromised, even temporarily, it can lead to URL blacklisting by security vendors, including Microsoft.
For advanced users, setting up a custom tracking domain can provide more control and insulate you from issues on shared domains. This involves configuring a subdomain of your own, like clicks.yourdomain.com, to handle the redirects. This way, the reputation of the tracking domain is tied directly to your practices.
Maintaining a healthy URL reputation
To prevent future issues, I recommend a proactive approach to managing your email program's URL health. This goes beyond just monitoring DNS-based blocklists and extends to the entire user journey after the click.
Regularly check your landing pages and any intermediate redirects for malicious content or signs of compromise. Use tools that scan for malware and phishing. If your website or any linked resource is infected, it will inevitably lead to blocking by security solutions like Microsoft Defender.
Maintaining a strong sender reputation isn't just about email authentication; it also encompasses the trustworthiness of your linked content. If your links consistently lead to clean, relevant content, your overall reputation with ISPs and security providers will improve, reducing the likelihood of future blocks.
Consider implementing DMARC, SPF, and DKIM for your sending domains. While these primarily affect email delivery and not URL reputation, they contribute significantly to your domain's overall trustworthiness and can indirectly influence how your links are perceived by security filters.
Views from the trenches
Here's how I think about preventing these issues:
Best practices
Scan regularly: Periodically scan your website and landing pages for malware and vulnerabilities.
Use custom tracking domains: If available from your ESP, configure custom tracking domains to control their reputation.
Monitor redirects: Ensure all redirects in your click paths are legitimate and not causing issues.
Maintain strong email authentication: Proper SPF, DKIM, and DMARC records bolster overall domain trust.
Common pitfalls
Ignoring shared domain issues: Assuming your links are safe just because your domain is clean, without considering ESP shared domains.
Outdated content: Linking to old, unmaintained landing pages that might contain vulnerabilities.
Complex redirect chains: Using too many redirects can appear suspicious to security filters.
Expert tips
Use a URL scanner: Before sending, manually test problematic links through public URL scanners for quick insights.
Monitor DMARC reports: While not directly for URL reputation, DMARC reports can flag issues that affect overall domain health.
Engage your ESP: Your Email Service Provider should be your first point of contact for tracking domain issues.
Final thoughts
In conclusion, when Microsoft Defender blocks your emails on CTA clicks, it's rarely about your domain being on a DNS blacklist. Instead, it points to the dynamic, real-time URL scanning capabilities of MDO's Safe Links feature.
The focus shifts to the reputation and integrity of the linked URLs, including any redirect chains or the final landing page content. By understanding this distinction and implementing proactive measures for URL health, you can significantly reduce these frustrating blocks and ensure your recipients can safely access your intended content.
Regular monitoring of your email campaigns, not just for delivery rates but also for user interaction with links, is crucial. This proactive approach ensures your email program remains healthy and effective.
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