Add inline images to emails with Content-ID (CID)

When sending emails with images, external links to web hosting or a CDN are often used. This works fine, but images can be blocked by email clients or fail to load when hosting is temporarily offline.

With Content-ID (CID), you send images inline with the email. They are directly included in the email itself, keeping them always visible without being blocked.

What is Content-ID?

Content-ID is a technique to embed images directly in emails. Instead of loading images from an external URL, you add the image as part of the email itself.

You use Content-ID for example for:

• Email signatures: logos and profile photos
• Newsletters: headers, product photos and visual elements

Perfect to use in both transactional emails and broadcast emails.

How does it work?

Using Content-ID is simple. You add an image as an attachment with a unique content_id, and then reference this image in your HTML with cid:your-content-id.

An example with our PHP SDK:

          $lettermint = new Lettermint\Lettermint('your-api-key');

$lettermint->email
    ->from('hello@yourdomain.com')
    ->to('customer@example.com')
    ->subject('Welcome to our service')
    ->html('
        <h1>Welcome!</h1>
        <img src="cid:company-logo" alt="Company Logo" />
        <p>Thanks for signing up.</p>
    ')
    ->attach('logo.png', base64_encode(), 'company-logo')
    ->send();

        

In the example, you can see that the src of the image references cid:company-logo. This corresponds to the content_id we provide with the attachment. This way the email client knows which image should be displayed where.

Besides PHP, Content-ID is also available in our other integrations such as Node.js, SMTP and via our API. All examples and possibilities can be found in the documentation.

Conclusion

With Content-ID, you are no longer dependent on external hosting. Images are sent directly and are always visible, even when email clients block external content.

• Images directly visible
• No hassle with hosting for external URLs
• Emails continue to work offline

For more information and integration examples, check out our documentation.