Summary
Images and PDF attachments significantly influence email deliverability. While images aren't strictly necessary, their size, ratio to text, optimization for mobile, use of alt text, and hotlinking can trigger spam filters or negatively impact user experience. PDFs, especially large, password-protected ones, or those with suspicious content (like embedded code or ".com" in the name), raise red flags. Excessive attachment sizes also violate server limits. Consistent branding is also key. Ultimately, a holistic approach focusing on content relevance, file size, secure coding practices, and user experience is vital.
Key findings
- Image Necessity: Images are not strictly required for email deliverability, but how they're used matters.
- Image/Text Ratio: Low text-to-image ratio increases spam likelihood.
- Image Optimization: Poorly optimized images hurt mobile user experience and engagement.
- PDF Risks: Large, password-protected PDFs or PDFs with embedded code are risky.
- Hotlinking Dangers: Hotlinking can cause deliverability issues if the server is unreliable.
- ALT text importance: Missing ALT text reduces accessibility and can hurt deliverability
- Overall Message Size: Exceeding total message size limits causes delivery failures.
- PDF Filename Issues: PDF filenames with ".com" trigger spam filters.
- Inconsistent Branding: Inconsistent branding can lead to distrust and spam reports.
Key considerations
- Optimize images: Reduce image file sizes and optimize them for mobile.
- Balance Text and Images: Ensure adequate text content to avoid spam filters.
- Secure PDF Content: Avoid embedding executable code or using password protection in PDFs.
- Choose Reliable Image Hosting: Use reliable image hosting or embed images (weighing privacy vs size tradeoffs).
- Provide descriptive alt text: Include descriptive ALT text for all images.
- Stay Within Size Limits: Stay within total message size limits set by email providers.
- Use Safe Filenames: Avoid using potentially problematic characters like '.com' in PDF filenames.
- Maintain Branding Consistency: Maintain consistent branding elements across all emails.
- Use safe attachments: Ensure all attachments are safe and relevant to the recipient to avoid being flagged as spam.
What email marketers say
9 marketer opinions
The use of images and PDF attachments in emails can significantly impact deliverability. Factors such as image size, text-to-image ratio, image optimization for mobile, use of alt text, and hotlinking practices all play a role. Similarly, large, password-protected, or executable-code-embedded PDFs can trigger spam filters. Maintaining consistent branding and avoiding suspicious content within attachments are also crucial for ensuring positive engagement and avoiding deliverability issues.
Key opinions
- Image Size: Large image sizes negatively impact deliverability by increasing loading times and spam scores.
- Text-to-Image Ratio: Emails with too many images or a single large image without sufficient text are prone to being flagged as spam.
- PDF Attachments: Unusually large or password-protected PDF attachments can raise red flags with spam filters.
- Hotlinking: Hotlinking images can negatively impact deliverability if the linked server is unreliable or the image is removed.
- Alt Text: Missing or irrelevant alt text for images can harm deliverability and accessibility.
- Executable Code in PDFs: Embedding executable code or macros within PDF attachments increases the risk of being flagged as spam.
- Mobile Optimization: Unoptimized images for mobile devices can lead to slow loading times and poor user experience, affecting engagement and deliverability.
- Consistent Branding: Inconsistent use of images and branding elements can lead to distrust and spam complaints, negatively impacting deliverability.
Key considerations
- Optimize Images: Always optimize images for web use to reduce file size and improve loading times.
- Maintain Text-to-Image Balance: Ensure a good balance between text and images to avoid triggering spam filters.
- Use Alt Text: Provide descriptive alt text for all images to improve accessibility and sender reputation.
- Avoid Suspicious PDF Content: Refrain from including executable code or password protection in PDF attachments.
- Mobile Optimization: Ensure images are optimized for mobile devices to provide a good user experience.
- Consistent Branding: Maintain consistent branding elements in all emails to build trust and avoid spam complaints.
- Attachment size limits: Be aware of any attachment size limits set by the email provider.
Marketer view
Email marketer from Email on Acid explains that missing or irrelevant alt text for images can harm deliverability and accessibility. Providing descriptive alt text ensures that users with image loading disabled or using screen readers can understand the content, improving engagement and sender reputation.
23 May 2022 - Email on Acid Blog
Marketer view
Email marketer from Stack Overflow shares that embedding executable code or macros within PDF attachments significantly increases the risk of being flagged as spam, leading to deliverability problems. Avoid including such elements in PDFs attached to emails.
11 Jul 2024 - Stack Overflow
What the experts say
5 expert opinions
Experts generally agree that while images are not required for email deliverability, their use, along with PDF attachments, can affect it. Simply including a PDF isn't inherently an issue, but longer PDFs, those containing malware, or those with suspicious names (like including ".com") can cause problems. Embedding images as attachments can offer privacy benefits by preventing tracking, but larger message sizes may impact deliverability. Finally, it's crucial to ensure all attachments, including PDFs, are safe, relevant, and free of misleading content to avoid being flagged as spam.
Key opinions
- Image Requirement: Images are not necessary for basic email deliverability.
- PDF Concerns: Longer PDFs or those potentially containing malware can cause deliverability issues.
- PDF Naming: PDF filenames including ".com" can be problematic.
- Embedded Images: Embedding images as attachments offers privacy benefits but increases message size.
- Attachment Safety: Attachments must be safe, relevant, and free of misleading content to avoid spam filters.
Key considerations
- Evaluate PDF Length: Keep PDFs as concise as possible to minimize potential issues.
- Avoid Suspicious Names: Refrain from using suspicious or misleading names for PDF files.
- Weigh Privacy vs. Size: Consider the trade-off between privacy benefits and increased message size when embedding images.
- Scan Attachments: Ensure all attachments are scanned for malware and are relevant to the recipient.
Expert view
Expert from Email Geeks explains there is no need for an image in email for better deliverability.
13 Mar 2025 - Email Geeks
Expert view
Expert from Email Geeks responds to concerns about PDFs, clarifying that while they can sometimes be a deliverability issue (especially longer PDFs that might hide malware), the mere presence of a PDF isn't inherently problematic.
2 Jan 2025 - Email Geeks
What the documentation says
4 technical articles
Technical documentation emphasizes that large images and PDF attachments significantly impact email deliverability. Exceeding server limits due to excessively large attachments can lead to delivery failures. A high volume of emails with primarily images (to bypass spam filters) can result in being flagged as spam. User-reported spam complaints about emails with large images or attachments can negatively affect sender reputation and thus, deliverability. Moreover, exceeding email message size limits, which images and PDFs contribute to, results in non-delivery.
Key findings
- Attachment Size Limits: Exceedingly large attachments can lead to delivery failures due to server limitations.
- Image Spam: A high volume of emails using primarily images to bypass text-based spam filters can harm deliverability.
- User Spam Reports: High spam rates reported by users for emails with large images or attachments can negatively impact sender reputation and deliverability.
- Message Size Limits: Exceeding email message size limits due to large images and PDFs results in non-delivery.
Key considerations
- Limit Attachment Sizes: Keep attachment sizes within reasonable limits to avoid server rejections.
- Balance Images with Text: Maintain a balanced ratio of text to images to avoid being flagged as image spam.
- Monitor Spam Complaints: Monitor and address user spam complaints to maintain a positive sender reputation.
- Stay Within Message Size Limits: Ensure total email message size, including attachments, stays within the provider's limits.
Technical article
Documentation from Google Postmaster Tools indicates that high spam rates reported by users for emails containing large images or attachments can negatively affect your sender reputation and lead to deliverability issues.
25 Mar 2023 - Google Postmaster Tools
Technical article
Documentation from RFC 5321 defines technical specifications for email and states that excessively large attachments can lead to delivery failures due to server limitations and potential triggering of spam filters.
2 Sep 2021 - RFC 5321
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