Summary
Linking to PDF files in emails can indirectly cause bounces due to security filters like Mimecast. The primary factors influencing this are the reputation of the hosting domain, the content of the PDF, and the accessibility of the link. Mimecast and other security filters scan the linked content and the linking domain, and may block emails if they detect a poor reputation, malicious content, or inaccessible resources. While linking bypasses attachment size limits, it doesn't circumvent content scrutiny. Adherence to RFC standards, maintaining a positive sender reputation, using reputable hosting, and scanning PDFs for malicious content are crucial for successful email delivery.
Key findings
- Domain Reputation: The reputation of the domain hosting the PDF significantly impacts deliverability. Blacklisted or untrusted domains can lead to email rejections.
- PDF Content: Spam filters scan the content within the PDF. Malicious content, spam-triggering keywords, or non-compliant elements can trigger filters.
- Accessibility Matters: If links are not reachable from certain geographic locations, this can raise suspicion and lead to bounces.
- Security Policies: Organizations configure security policies to block specific content types or URLs, impacting email delivery.
- Linking vs Attachment: While linking circumvents size limits, it doesn't guarantee better deliverability; the link is still subject to content scanning.
Key considerations
- Reputable Hosting: Use reputable and trusted hosting services for PDFs.
- Content Scanning: Proactively scan PDFs for malicious content, spam triggers, and non-compliant elements.
- Sender Reputation: Maintain a positive sender reputation by following email best practices.
- Test Accessibility: Ensure PDFs are accessible from various geographic locations.
- RFC Compliance: Adhere to RFC specifications for email formatting.
What email marketers say
11 marketer opinions
Linking to PDFs in emails does not directly cause bounces, but several factors can trigger spam filters and impact deliverability. These factors include the reputation of the domain hosting the PDF, the content within the PDF itself, and whether the links are reachable from various locations. Security filters such as Mimecast may block emails with links to untrusted or blacklisted sites. Link shorteners can also negatively impact deliverability. Using reputable hosting services, building a positive sender reputation, and ensuring clean, malware-free content are crucial.
Key opinions
- Domain Reputation: The reputation of the domain hosting the PDF is a significant factor. Links to untrusted or blacklisted domains are more likely to be flagged as spam.
- PDF Content: The content within the PDF is scanned and can trigger spam filters if it contains suspicious or malicious content.
- Accessibility: If the linked PDF is not reachable from certain locations (e.g., due to regional restrictions), it can cause bounces as security scanners may interpret this as suspicious activity.
- Attachment vs Link: While linking can bypass some attachment filters, the content is still subject to security checks. Using a link may not guarantee better deliverability than attaching the PDF.
Key considerations
- Use Reputable Hosting: Host PDFs on reputable and trusted domains to avoid being flagged as suspicious.
- Content Scrutiny: Ensure the PDF content is clean, free of malware, and does not contain spam-triggering keywords or phrases.
- Monitor Sender Reputation: Maintain a positive sender reputation by adhering to email best practices and avoiding practices that could damage your sender score.
- Avoid Link Shorteners: Using link shorteners can negatively impact deliverability. Prefer using the full, direct link to the PDF where possible.
- Accessibility Testing: Verify that the PDF is accessible from various geographic locations to prevent accessibility-related bounces.
Marketer view
Email marketer from EmailGeek Forums User PDFExpert states that while PDF attachments can cause bounces linking to files also can do the same if the file is on a unsafe server.
26 Sep 2022 - EmailGeek Forums
Marketer view
Email marketer from EmailToolTester says that if you link to a blocked or blacklisted site, then yes that is an issue. But generally linking to a PDF is not as big an issue as it used to be.
4 Jul 2023 - EmailToolTester
What the experts say
3 expert opinions
Experts agree that linking to PDFs in emails can impact deliverability, primarily due to domain reputation and content security concerns. If the linked domain has a poor reputation or the PDF contains suspicious content, spam filters, including Mimecast, may flag the email. Additionally, blocking content analysis gateways from accessing the linked content can be interpreted as malicious activity and lead to rejection.
Key opinions
- Domain Reputation Matters: A poor reputation of the linked domain can lead to emails being flagged as spam.
- Content Analysis is Crucial: Suspicious or harmful content within the PDF can trigger spam filters.
- Content Blocking Signals Malware: Preventing content analysis gateways from accessing the linked content can be a sign of malicious intent.
- Generic Rejections Lack Detail: Mimecast's generic rejections don't provide detailed reasons, making troubleshooting more challenging.
Key considerations
- Monitor Domain Reputation: Regularly check the reputation of domains used for hosting PDFs.
- Scan PDF Content: Thoroughly scan PDFs for potentially harmful content before linking to them.
- Avoid Blocking Gateways: Ensure content analysis gateways can access linked PDFs to avoid being flagged as malicious.
Expert view
Expert from Word to the Wise responds sharing that including links to domains with low reputations, or to content that is identified as spam can negatively affect email placement and deliverability, including linking to a PDF.
6 Feb 2022 - Word to the Wise
Expert view
Expert from Spam Resource shares that linking to PDFs can be problematic if the linked domain has a poor reputation or if the PDF itself contains suspicious content. Spam filters may flag these links as potentially harmful.
30 Jul 2024 - Spam Resource
What the documentation says
4 technical articles
Technical documentation from various sources indicates that while linking to PDFs in emails can circumvent attachment size limits and some attachment filtering, it does not guarantee deliverability. Security policies configured by organizations, adherence to email format standards (RFC), and content scanning tools used by providers like Google Workspace and Microsoft Exchange Online Protection can still flag linked PDFs as malicious or non-compliant, leading to email rejections or filtering.
Key findings
- Security Policy Enforcement: Organizations can configure security policies that block specific content types or URLs, affecting linked PDFs.
- RFC Compliance: Emails not adhering to RFC specifications for formatting attachments and links can trigger security filters.
- Content Scanning Persists: Linking to a PDF does not prevent content scanning by email providers; malicious content can still lead to filtering.
- Size Limit Circumvention: Linking to a PDF allows bypassing attachment size limits imposed by email providers like Google Workspace.
Key considerations
- Understand Recipient Policies: Be aware of the security policies of recipient organizations that may impact linked PDFs.
- Ensure RFC Compliance: Adhere to RFC specifications for email formatting to avoid triggering filters.
- Scan PDF Content Proactively: Thoroughly scan PDF content for malicious elements before including links in emails.
- Optimize PDF Size: While linking bypasses size limits, optimize PDF size to improve user experience.
Technical article
Documentation from Google Workspace answers that Google Workspace has limits on attachment and message sizes. Linking to a PDF avoids the size limitations but the link could be checked by security tools.
12 Jul 2022 - Google Workspace
Technical article
Documentation from RFC Editor states that RFC specifications define standards for email formats, including how attachments and links should be handled. Non-compliant formatting may trigger security filters.
24 Oct 2023 - RFC Editor
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