What this RFC defines
RFC 2392 defines two URI schemes for referencing parts of email messages: cid: (Content-ID) for referencing a specific MIME part within the same message, and mid: (Message-ID) for referencing an entire email message. These URIs enable HTML email to reference embedded images by their Content-ID rather than external URLs.
Where you see it in practice
When an HTML email contains an embedded image stored as a MIME attachment within the message itself (rather than hosted on a web server), the img src attribute uses a cid: URI like src=”cid:image001@example.com”. The MIME part containing the image has a matching Content-ID: <image001@example.com> header. This technique avoids external image tracking pixels and allows images to display even without network access.
How it connects to other RFCs
RFC 2392 works within the MIME framework defined by RFC 2045 and RFC 2046 (multipart/related). The cid: scheme references Content-ID values defined in RFC 2045. The mid: scheme references Message-ID values defined in RFC 5322. Together they provide a linking mechanism between email parts.
Current status
RFC 2392 is a current standard, published August 1998. CID-referenced embedded images remain a common technique in HTML email, particularly for corporate email signatures and newsletters that want images to display without external requests. Email client support is universal.
CID and MID URLs
RFC 2392 defined two URL schemes for referring to email content: cid: (Content-ID) URLs point to a specific MIME part within the current message, and mid: URLs point to another message by Message-ID. The cid: scheme is the mechanism that lets HTML email display inline images: the img tag src=”cid:image1@example.com” tells the mail client to look for a MIME part with Content-ID: . Without this, HTML email would need to reference external image URLs, which triggers privacy warnings.
Inline image implementation
Every HTML email marketing platform, transactional email system, and mail client that renders inline images uses cid: URLs per RFC 2392. The construction requires several coordinated pieces: the HTML body references cid:UNIQUE-ID, the corresponding MIME part sets Content-ID: , the part uses Content-Disposition: inline, and the multipart type is multipart/related (RFC 2387). Any of these pieces missing or misconfigured causes broken images or unexpected attachment prompts.
MID URLs in practice
The mid: URL scheme is far less common than cid:. It sees limited use in mail archives (linking one message to another by Message-ID) and in message search interfaces. Some enterprise systems use mid: URLs for compliance references (linking a policy notification to the original triggering message). For consumer email use, mid: URLs are largely unused; cid: URLs dominate.
RFC 2392 (August 1998) defines the cid: and mid: URL schemes: syntax for referencing MIME body parts by Content-ID (cid:) and messages by Message-ID (mid:). Foundation for HTML emails with embedded images (the <img src="cid:logo123@example.com"> pattern). Nearly universal in transactional and marketing email; enables logos, product images, and inline graphics without external hosting. Companion to Content-Disposition.
RFC 2392 at a glance
| Aspect | Detail |
|---|---|
| Purpose | URL syntax for referencing MIME parts by Content-ID and messages by Message-ID |
| cid: scheme | Reference a MIME part in the same message |
| mid: scheme | Reference a specific message (rare use) |
| Companion header | Content-ID (identifies the referenced part) |
| Related MIME type | multipart/related (RFC 2046) |
| Published | August 1998 |
HTML email with embedded images
The mid: URL scheme (rare)
Common cid: implementation mistakes
Content-ID: <logo@example.com>. The cid: URL does NOT include them: cid:logo@example.com. Emitting angle brackets in the URL breaks the reference.Content-ID: <Logo123@example.com> and cid:logo123@example.com may fail to resolve. Use consistent case; lowercase is convention.Related standards and further reading
- RFC 3986: URI syntax (base URL specification)
- RFC 5322: Message Format (Message-ID defined here)
- Message-ID Header: identifier referenced by mid: URLs
- Content-Type and MIME Structure: multipart wrapper
Frequently asked questions
Should I use cid: URLs or hosted images in HTML email?
Depends on trade-offs. cid: URLs embed the image in the message (guaranteed to display, larger message size, works offline). Hosted images link to a URL (smaller message, tracking pixels possible, may not display if images are blocked). Modern practice: use cid: for critical branding elements (logos), hosted for content images (product photos, hero images). Many bulk senders use hosted only, accepting the blocked-images cost for reduced message size.
Why do embedded images sometimes not display?
Common causes: (1) cid: URL does not match Content-ID header (case mismatch, angle brackets, typo), (2) wrong MIME wrapper (multipart/mixed instead of multipart/related), (3) mail client blocks embedded images per user preference, (4) image part missing from message. Test with mail-tester.com or similar tools to catch structure issues before deploying.
Can I use cid: URLs in CSS?
Yes, in inline style attributes: <div style="background-image: url(cid:pattern@example.com)">. Mail client CSS support is limited; test across clients. External stylesheets are typically not supported in email regardless. Inline styles are the standard approach.
Are mid: URLs used for anything in modern email?
Rarely. Consumer mail clients typically do not follow mid: links. Occasional use in mail archive systems (reference to a specific archived message), compliance/legal reporting (linking policy notices to originating messages), and some enterprise workflow tools. For consumer email use, mid: URLs are largely unused; cid: URLs dominate for embedded content.
What is the security model for cid: URLs?
Local to the message; no external resource loaded. A cid: URL resolves within the current message’s parts and cannot reach the network. Safe from external tracking or content-injection perspectives. Contrast with hosted images (external URL): those can track opens via image loads. cid: images do not enable open tracking; that is a trade-off many bulk senders explicitly accept by using hosted.
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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