How do CID-referenced images work?
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You're sending an email with a logo, and you want it to display even if the recipient's email client blocks external images. That's where CID-referenced images come in. CID stands for Content-ID, and it's a way to embed images directly inside the email itself instead of linking to them from a server.
But Here's what happens: your email gets built as a MIME multipart message with two parts. The first part is your HTML body. The second part is your image file, and it gets labeled with a Content-ID header (like Content-ID: <logo123>). Your HTML then references that image using a special cid: URL: <img src="cid:logo123">. The email client connects those references and displays the image directly.
The tradeoff is real though. CID images work great in Outlook and corporate email systems, but they're not as reliable across all clients. Most marketers stick with externally hosted images for better compatibility. Want to see how your images actually render? Test your designs with an email preview tool across different clients before you decide on CID.
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