Do images in email and PDF attachments affect email deliverability?

Key findings

• Direct impact is limited: Simply including images or PDFs does not inherently harm deliverability. Many legitimate emails, especially marketing and transactional ones, use them without issues.
• File size matters: Large images or multiple, heavy PDF attachments can significantly increase email size, leading to slower loading times, potential rejections, or triggering spam filters. This is a common issue that impacts email file size and MIME types affecting deliverability.
• Image-to-text ratio: Emails that are predominantly images with little or no text can be flagged as suspicious, as spammers often use this technique to bypass text-based filters. This highlights the importance of image-to-text ratio for deliverability.
• PDF specific risks: PDFs can carry malware, so some spam filters are more scrutinizing of them, especially if they are unusually large or contain suspicious elements (like executable content). Certain filename patterns (e.g., those containing '.com') can also trigger bounces.

Key considerations

• Optimize image sizes: Compress images to reduce file size without compromising quality. This helps emails load quickly and reduces the overall email size. You can learn more about compressing images for email.
• Balance content: Ensure a healthy mix of text and images. Don't rely solely on images to convey your message; always include a plain text alternative and descriptive alt text.
• Consider linking for PDFs: Instead of attaching large PDFs, upload them to a server and provide a direct link in the email. This reduces email size and bypasses attachment-related filtering concerns. This practice is often seen as a better alternative to PDF attachments.
• Monitor deliverability: Regularly test your email campaigns and monitor your deliverability rates, especially after making changes to your content or attachment practices. You can also run an email deliverability test.

Key opinions

• Perceived penalties: Some marketers observe that certain email analysis tools might penalize emails for lacking images, suggesting an unspoken expectation from providers for rich content, especially in marketing emails.
• Content type distinction: Emails with multiple links and no images might be perceived as less personal by email providers, potentially affecting their filtering decisions compared to visually rich marketing or system emails.
• Attachment size and quantity: Marketers frequently encounter issues when sending emails with numerous or very large attachments, leading to bounces or spam flagging. This includes customer-generated content like large image collections.
• Filename issues: Unusual elements in attachment filenames, such as a company name containing '.com', have been observed to cause hard bounces from major email providers like Gmail.
• Skepticism about general advice: Some marketers express caution regarding broad advice (e.g., that all PDFs are bad) given that many reputable retailers successfully send emails with PDF invoices and receipts regularly.

Key considerations

• A/B test content: Experiment with different content compositions (image vs. text heavy, attachment vs. link) to understand what performs best for your specific audience and sending patterns. This ties into how image to text ratio affects deliverability.
• Set attachment limits: Implement internal policies or system limits on the number and size of attachments to prevent deliverability issues caused by excessive email payload.
• Review filename conventions: Be cautious of unusual characters or patterns in attachment filenames that could trigger automated spam filters or cause hard bounces. This is a common best practice when handling PDF attachments and deliverability.
• Prioritize recipient experience: Even if deliverable, large emails with heavy images or attachments can negatively impact user experience due to slow loading times, which can indirectly affect engagement and sender reputation. Mailchimp highlights that large attachments can trigger spam filters.

Key opinions

• No inherent need for images: Experts confirm that images are not a prerequisite for good deliverability. Many plain-text emails deliver perfectly fine.
• Contextual spam susceptibility: Emails containing multiple links but no images can be more prone to being marked as spam. This risk is amplified when combined with existing domain or IP reputation challenges, especially if the message is multipart.
• Score calculation complexity: Deliverability scores from analysis tools are often based on a combination of content factors and the sender's domain/IP reputation, not just isolated elements.
• PDF filtering nuances: While a PDF attachment itself isn't a direct deliverability concern, filters do scrutinize PDFs for potential malware, especially longer or unusually constructed ones. Therefore, it's not simply being a PDF that's the issue, but what might be contained within it.
• Discrepancies in advice: Experts may express skepticism about generalized do not attach PDFs advice, pointing out that many high-volume senders successfully use PDF attachments (e.g., for invoices) regularly. This suggests a more complex filtering logic beyond simple attachment presence.

Key considerations

• Monitor reputation alongside content: Recognize that content elements like images and links are evaluated in conjunction with your sending IP and domain reputation. A good reputation can mitigate minor content flags. Learn more about why your emails fail.
• Understand multipart messages: Be aware that the multipart nature of emails (containing both HTML and plain text parts) can influence how content, including the absence of images, is perceived by spam filters. Understanding how self-hosting images affects deliverability is important.
• Beware of excessive attachments: While a single small PDF may be fine, experts strongly advise against sending numerous or extremely large attachments, as this pattern is a clear red flag for deliverability issues.
• Consider the type of email: Transactional emails with expected attachments (like invoices) are generally treated differently than marketing emails with large, unnecessary attachments. Kickbox discusses the impact of images on email deliverability.

Key findings

• Email loading times: Large image files directly increase the overall size of an email, which can lead to slower loading times for recipients. This impacts user experience and can contribute to emails being flagged or truncated.
• Image-only emails: Emails consisting solely of images with minimal or no text are often viewed with suspicion by spam filters. This is because spammers frequently use this technique to embed malicious content or obscure text that would otherwise be caught by filters. This directly affects whether image-only emails are bad for deliverability.
• Attachment size and quantity: Sending multiple attachments, especially if they are large image files or PDFs, can significantly impact deliverability. This can trigger spam filters or even cause emails to bounce if they exceed size limits.
• Embedding methods: The method of embedding images (e.g., CID, HTML inline, external hosting) can affect email size and how clients render the content. External hosting generally results in smaller email sizes, as the images are fetched dynamically.

Key considerations

• Image compression: Always compress images for email to minimize file size. This practice helps ensure faster loading and reduces the overall email payload. ShortPixel Blog offers easy tricks for tiny image sizes.
• Alt text usage: Include descriptive alt text for all images. This serves as a fallback if images don't load and is crucial for accessibility, also potentially aiding deliverability by providing textual context to filters.
• HTML and plain text versions: Always send multipart emails with both HTML and plain text versions. The plain text version ensures readability even if images are blocked or not supported. This relates to how images can cause emails to go to spam.
• Link to PDFs, don't attach: For documents like PDFs, providing a link to an externally hosted file is generally safer and more efficient than direct attachments, reducing email size and bypassing attachment-specific filters. Understand the impact of direct PDF download links.