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
Related Glossary Terms
Dark Mode
Dark mode in email refers to how email clients render messages when the recipient has dark mode enabled. It affects colour display, image rendering, and readability. Designers use CSS media queries and hybrid coding techniques to control the dark mode appearance.
Drag-and-Drop Email Editor
A drag-and-drop email editor is a visual interface that allows marketers to build email layouts by dragging content blocks into place without writing HTML or CSS code.
Email Accessibility Guidelines
Email accessibility guidelines ensure emails are usable by people with disabilities, following WCAG standards for screen readers, colour contrast, keyboard navigation, and readable content.
Email Accessibility
Email accessibility ensures marketing emails are usable by people with disabilities, including those using screen readers, keyboard navigation, or assistive technologies. It follows WCAG 2.1 guidelines for structure, contrast, semantics, and descriptive content.
Email Body
The email body is the main content area of an email message, containing the visible text, images, and structural elements that communicate the message to the recipient.
Border
Border is a CSS styling property used in email design to define visible lines around elements such as tables, images, buttons, and content sections, contributing to visual structure and hierarchy.
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.