How do CID-referenced images work?

CID (Content-ID) images are embedded directly in the email as MIME attachments and referenced in HTML using a special cid: URL scheme. Example: <img src="cid:logo123"> where "logo123" corresponds to a Content-ID header in the attached image part.

How it works: The image travels with the email as a multipart attachment. The email client recognizes the CID reference and displays the attached image inline. This bypasses external image loading. The uimage is already present.

Use cases and trade-offs: Pros: Images display immediately without blocking prompts, survive forwarding intact, work offline. Cons: Significantly increases email size, inconsistent rendering across clients, can trigger spam filters, complicates tracking. CID embedding is niche. Standard practice is external hosting; CID is mainly for specific B2B or transactional scenarios where guaranteed image display outweighs size concerns.

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