Inline image
CIDAn image embedded directly into an HTML email body using a Content-ID (cid:) reference, rather than attached as a separate downloadable file. The image data is stored as a MIME part within the same message.
Inline images use the multipart/related MIME structure. The HTML body part references an image with a src attribute like src="cid:[email protected]", and a corresponding MIME part in the same message carries the actual image data with a Content-ID header of <[email protected]>. This allows formatted emails to include logos, diagrams, or decorative images that appear embedded in the layout without requiring the recipient to be online.
Inline images differ from remote images, which use normal HTTP URLs (src="https://example.com/logo.png") and are fetched from an external server when the message is opened. Remote images are a known privacy concern because the server can track when and by whom the image was requested (a "tracking pixel" technique). Privacy-conscious mail clients block remote images by default.
Mbox Viewer displays inline images by resolving cid: references to the corresponding MIME parts within the message, so formatted emails render correctly in offline viewing. Remote images are blocked to protect privacy, consistent with the app's local-only, read-only design.
Related terms
A file — such as a PDF, image, or spreadsheet — embedded in an email message and encoded as a MIME part, separate from the message body, intended for the recipient to save or open.
An email message whose body is formatted with HTML and CSS, allowing rich typography, layout, colors, and images. Most modern email is HTML, but privacy and security considerations require careful rendering.