Definition
An email attachment is a file — PDF, image, document, or other format — sent alongside the email message. In marketing email, attachments are controversial: they can trigger spam filters, increase email size, create rendering issues across devices, and pose security risks for recipients who are not expecting files.
The general best practice for marketing emails is to avoid attachments entirely. Instead of attaching a PDF, host the file on your website and link to it. Instead of embedding a large image, host it and reference the URL. This approach improves deliverability, reduces email size, and gives you engagement tracking through click data.
Attachment Risks in Marketing Email
| Risk | Impact |
|---|---|
| Spam filter triggering | Attachments increase spam score |
| File size limits | Most ESPs limit emails to 10–25MB total |
| Security concerns | Recipients are wary of unknown attachments |
| Mobile rendering | Attachments difficult to open on mobile |
| Outlook rendering | Attachments can break layout in Outlook |
| Tracking | Cannot track if attachment was opened (vs click on link) |
Best Practices
- Do not attach files to marketing emails — use hosted links instead
- For PDFs, whitepapers, and reports, host on your website and link prominently
- For images, use hosted image URLs rather than image attachments
- Transactional emails may include attachments (invoices, tickets) when expected by the recipient
- If you must attach files, use common formats (PDF rather than DOCX) and keep sizes small
- Notify recipients that the email contains an attachment in the subject line or preview text
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Related Glossary Terms
Above the Fold in Email
Above the fold in email refers to the content visible to a recipient before they scroll, a critical area for first impressions and primary calls to action.
Accessible CTA (Email)
An accessible CTA is a button or link coded with sufficient colour contrast, visible focus states, descriptive screen-reader text, and a minimum touch target of 44×44 pixels.
Adaptive Design
Adaptive design in email uses predefined layouts that adjust to specific device or client widths, rather than fluidly resizing to every possible screen.
AI-Generated Content in Email
AI-generated content in email is copy, images, code or subject lines produced by artificial intelligence tools to speed up campaign production and testing.
Alt Text
Alt text is the written alternative to an image in an email, displayed when images are blocked, slow to load, or consumed by screen readers.
Autoplay Video in Email
Autoplay video in email refers to the limited ability to play video content automatically when an email is opened, using specific client-supported techniques.
Frequently Asked Questions
Spam filters treat attachments with suspicion because malware and phishing emails frequently use attachments. Even legitimate attachments increase your spam score. Additionally, attachments increase email size, which can trigger size-based filtering at the mailbox provider level.
Host your file on your website, cloud storage, or a content delivery network. Include a clear, recognisable link or button in the email that recipients can click to access or download the file. This approach avoids deliverability issues, allows click tracking, and works better on mobile devices where saving attachments can be difficult.