Can linking to PDF files in email cause bounces due to Mimecast or other security filters?
Michael Ko
Co-founder & CEO, Suped
Published 5 Aug 2025
Updated 19 Aug 2025
7 min read
The question of whether linking to PDF files in emails can cause bounces, especially due to advanced security filters like Mimecast, is a common concern for email marketers and IT professionals alike. While simply linking to a PDF isn't inherently problematic, certain factors can trigger security measures, leading to deliverability issues.
Email security gateways are designed to protect recipients from malicious content, and they cast a wide net. This includes scrutinizing not just attachments, but also links within the email body. When these filters detect anything suspicious, they can block the email, send it to spam, or even cause it to bounce, impacting your overall email deliverability. This is why it's crucial to understand their mechanisms and adapt your sending practices accordingly.
How email security filters scrutinize links
Email security filters, including robust platforms like Mimecast, employ sophisticated techniques to analyze email content, attachments, and URLs. Their primary goal is to identify and neutralize threats such as malware, phishing attempts, and spam before they reach the recipient's inbox. This often involves real-time scanning and dynamic URL rewriting.
When an email contains a link, Mimecast's URL protection service often rewrites the original URL to point to a Mimecast server. This allows them to scan the linked content for malicious indicators in real-time, every time the link is clicked. If the content, including a PDF, is deemed suspicious or dangerous, access to it can be blocked, or the email itself might be rejected.
The challenge arises when the linked content is perceived as high-risk. While a PDF document is generally harmless, it can be a vector for malware if exploited. Security filters look for various signals, including the reputation of the hosting domain, the history of the IP address, and any anomalies within the file itself. Therefore, a bounce message indicating a security policy violation is often a sign that the link, or the content behind it, failed one of these checks. More general bounce messages can be further investigated for clues.
The risk of linking to untrusted or public storage domains
One of the most common reasons for linking to PDF files to cause issues is when these files are hosted on untrusted or public storage domains, such as Google storage buckets or other generic cloud hosting services. While convenient, these platforms are often abused by spammers and malicious actors to host illicit content, which makes security filters inherently wary of links pointing to them.
When filters encounter a link to a domain with a poor or unknown reputation, they often err on the side of caution. This is why even a legitimate PDF on a suspicious domain can trigger a block. The filter can't easily distinguish between a harmless PDF and a malicious one if the hosting environment itself is questionable. The URL being unreachable from the filter's scanning location can also be a red flag, leading to a bounce or block.
In fact, linking to direct PDF download links can contribute to negative sender reputation if not managed carefully. The context surrounding the link is as important as the link itself. An email with multiple suspicious links or a history of low engagement from the sending domain combined with a generic cloud storage link will almost certainly face heightened scrutiny from security measures, resulting in bounces or messages ending up in spam folders.
Common causes of PDF link-related bounces
When an email bounces with a message like "554 Email rejected due to security policies," as observed in some Mimecast rejections, it indicates that the filter detected something it considered a threat. This is a generic but clear signal. For PDF links, several factors can lead to this type of rejection.
Specific Mimecast policies might be configured to block or strip certain types of attachments or links based on file type, size, or hosting domain. If a PDF link triggers a rule, the email might be quarantined or bounced. Furthermore, if the linked PDF contains any embedded scripts, macros, or elements that could be exploited, even if dormant, this can also raise a red flag. Filters are trained to identify patterns associated with malware, and PDFs can be sophisticated vessels for such content.
Unreachable URLs or links that redirect multiple times can also be problematic. Security filters attempt to follow and scan the ultimate destination of a link. If the destination is inaccessible during the scanning process, or if the redirects appear suspicious, the link may be flagged. This is particularly relevant if the host server (e.g., a Google storage bucket) has regional access restrictions or temporary outages that prevent the scanner from reaching the file.
Best practices for linking PDFs in emails
To mitigate the risk of bounces and ensure smooth delivery when linking to PDF files, several best practices should be adopted. Firstly, always host your PDF documents on your own trusted domain, or a reputable Content Delivery Network (CDN) that you control and that has a strong sender reputation. This eliminates the uncertainty associated with generic public storage domains.
Instead of directly linking to the PDF, consider linking to a dedicated landing page where users can then choose to download or view the document. This provides an additional layer of security and trust, as the security filter scans the landing page first, which is typically on your well-reputed domain. It also offers a better user experience and allows for tracking engagement.
Finally, ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured. Strong authentication signals to mail servers and security filters that your emails are legitimate and from an authorized source, significantly improving your sender reputation and overall deliverability. This foundational aspect of email security helps build trust with recipients and their security gateways.
Ensuring reliable PDF link delivery
While linking to PDF files in emails can indeed cause bounces, particularly due to vigilant security filters like Mimecast, it is not an insurmountable problem. The key lies in understanding the reasons behind these blocks and implementing best practices that build trust with email security gateways. By controlling where your PDFs are hosted, presenting links in a secure and transparent manner, and maintaining strong email authentication, you can significantly reduce the risk of your emails being flagged or bounced.
Proactive monitoring of your email deliverability and understanding bounce messages are also crucial. When you encounter a bounce, especially a generic security policy rejection, investigate the linked URL's accessibility and reputation. Adapting your strategy to align with security best practices will ensure your important documents reach their intended recipients without unnecessary interruptions.
Views from the trenches
Best practices
Always host your PDF files on a reputable domain that you control, rather than generic public cloud storage or shared services.
Use a dedicated landing page for PDF downloads, allowing security scanners to check your trusted domain first before the actual PDF download link.
Regularly check the accessibility of your linked URLs from various geographical locations and networks to ensure filters can reach them.
Maintain high sender reputation by adhering to email authentication standards and monitoring your sending practices.
Common pitfalls
Linking directly to public cloud storage URLs (e.g., Google Drive, Dropbox) which are often associated with spam and malware.
Using URLs that are unreachable or have inconsistent availability, causing security filters to flag them as suspicious.
Not maintaining proper email authentication (SPF, DKIM, DMARC), which reduces trust with recipient mail servers.
Ignoring bounce messages from security filters, preventing you from identifying and addressing underlying link issues.
Expert tips
Ensure your DNS records for SPF, DKIM, and DMARC are robust and correctly configured to authenticate your emails effectively.
Scan your PDF files for any hidden scripts, macros, or vulnerabilities that might be flagged by advanced security filters.
Segment your audience and test PDF link deliverability with a small group before sending to a larger list.
Consider compressing large PDF files to reduce download times, which can also indirectly affect how filters process links.
Marketer view
A marketer from Email Geeks says that blocking content analysis gateways from seeing hostile content, while allowing recipients to see it, is a tactic commonly associated with malware.
2024-11-26 - Email Geeks
Marketer view
A marketer from Email Geeks notes that a generic rejection message from Mimecast, like "554 Email rejected due to security policies," means it dislikes something without providing specific details.
Mimecast, employ sophisticated techniques to analyze email content, attachments, and URLs. Their primary goal is to identify and neutralize threats such as malware, phishing attempts, and spam before they reach the recipient's inbox. This often involves real-time scanning and dynamic URL rewriting.
Google storage bucket) has regional access restrictions or temporary outages that prevent the scanner from reaching the file.
