Do PDF attachments negatively impact email deliverability and what are the best practices?

Key findings

• Spam risk: Email clients often flag attachments as potential spam or malware threats, regardless of the file type. This can significantly reduce inbox placement rates.
• Resource consumption: Large attachments, especially PDFs, consume substantial server resources for sending and storage, impacting infrastructure performance and costs, particularly for mass mailings.
• User experience: Recipients may experience slow download times, especially on mobile devices or with limited internet access. Attachments also make it harder to track engagement compared to links.
• Lack of control: Once an attachment is sent, the sender has no control over it. If the content needs to be updated, recipients are left with outdated versions.
• Transactional vs. marketing: While transactional emails (e.g., invoices) might occasionally include attachments, marketing emails should almost always avoid them to maintain strong email deliverability.

Key considerations

• Link to hosted files: The best practice is to host PDFs on a secure, publicly accessible server and include a link in your email instead of attaching the file directly. This improves deliverability and user experience. For more on this, read our guide on linking to PDFs in email marketing.
• Pre-notify recipients: If an attachment is absolutely necessary, inform the recipient beforehand. This builds trust and reduces the likelihood of the email being marked as spam, as highlighted by ISIPP in their blog.
• Monitor deliverability: Regularly monitor your email deliverability rates and sender reputation. This helps identify if attachments are negatively impacting your inbox placement.
• Optimize file size: If linking is not an option for essential attachments, keep file sizes as small as possible to minimize impact on loading times and spam filters.
• Content alternatives: Consider if the information in the PDF can be presented directly within the email body, as HTML content is generally safer for deliverability.

Key opinions

• Definite negative impact: Many marketers view the negative impact of PDF attachments on deliverability as a certainty, not just a possibility.
• Spam flag: Attachments, particularly PDFs, are seen as red flags by email clients because they could potentially contain malware or viruses.
• ESP limitations: Some Email Service Providers (ESPs) actively discourage or do not offer the functionality to attach files, reinforcing the idea that it's a bad practice.
• User experience concerns: Marketers also highlight the poor user experience of attachments, especially for 'living documents' that are subject to updates, leading to cluttered inboxes with outdated versions.
• No engagement tracking: Attaching a PDF means losing the ability to track engagement (like clicks) which is crucial for optimizing campaigns, unlike when linking to a hosted file.

Key considerations

• Host and link: The preferred alternative is always to host the PDF on a public server and link to it from within the email. This allows for better control and tracking, as also noted by Mailgun’s deliverability tips.
• Transactional vs. bulk: While transactional emails might occasionally include attachments for specific, high-importance cases, it is generally advised against for marketing or bulk sends to avoid emails going to spam.
• Legal/compliance exceptions: In rare instances where legal or regulatory requirements necessitate attachments (e.g., temporary ID cards from insurance), marketers may explore alternative SMTP sending methods outside their primary ESP.
• Optimize processes: Instead of merely reducing file size, marketers should advocate for fully optimizing processes to move away from direct attachments for improved deliverability and user experience. Check our guide on email file size.

Key opinions

• Increased spam scoring: Attachments often add to the spam score of an email, even if the content is legitimate. This can push emails into spam folders or blocklists (blacklists).
• Infrastructure strain: Large attachments, especially in bulk, can strain sending infrastructure, leading to slower sending speeds, higher CPU usage, and increased cloud costs due to DKIM signing and data transfer.
• Security implications: Mailbox providers are highly cautious of attachments due to the risk of malware and phishing attempts, leading to stricter filtering.
• Reduced engagement signals: Emails with attachments may see lower open and click-through rates as recipients are hesitant to download unknown files, impacting sender reputation.
• Environmental impact: Sending large attachments unnecessarily contributes to a higher carbon footprint due to data transmission and storage, an often-overlooked aspect.

Key considerations

• Prioritize linking: Always recommend hosting files externally and providing a link. This improves deliverability, allows for tracking, and ensures recipients always access the most current version.
• Assess necessity: Rethink if the content truly requires a PDF. Often, information can be presented within the email body or on a linked landing page for better engagement and deliverability, which helps avoid email deliverability issues.
• Monitor blocklist (blacklist) status: Keep a close eye on your IP and domain's blocklist status. Frequent attachment usage can contribute to being listed, impacting your sending reputation. Regular blocklist monitoring is key.
• Segment sending: If attachments are unavoidable for specific segments, consider isolating these sends or using a separate sending IP to protect your main sender reputation.
• Education: Educate internal teams and stakeholders on the risks associated with attachments and the benefits of linking, to foster better email practices across the organization.

Key findings

• Spam triggers: Documentation explicitly states that attachments are a known trigger for spam filters due to their association with malicious content and phishing attempts.
• File size limits: Large file sizes, even for PDFs, can cause deliverability issues and are often cited as a reason for email rejection by mail servers.
• Security risks: Attachments pose a security risk. They can carry viruses or malware, leading mailbox providers to scrutinize emails with attachments more heavily.
• Reduced performance: The processing required for attachments, such as DKIM signing, can impact sending infrastructure performance and lead to delays, especially for bulk sends.
• Limited engagement insights: Email marketing platforms often highlight that attachments prevent tracking of critical engagement metrics like clicks, unlike linked content.

Key considerations

• Embrace linking: The documentation strongly recommends embedding links to hosted PDFs or other files rather than attaching them directly. This is a primary best practice. You can learn more about linking to PDFs and deliverability.
• Content optimization: Consider restructuring content to fit within the email body or on a dedicated landing page to avoid attachments entirely, improving both deliverability and user experience. This is part of broad email testing best practices.
• Transactional caution: Even for transactional emails where attachments might seem necessary, documentation often advises extreme caution and suggests alternative methods where possible, or to ensure very small file sizes.
• Pre-delivery testing: Some documentation advises testing emails with attachments thoroughly across various mailbox providers to gauge the potential impact on inbox placement before a large send.