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RFC 6152 / SMTP 8-BIT MIME TRANSPORT

Separate body support from other capabilities

05

8BITMIME is a body-transport extension. It does not by itself authorize internationalized envelope addresses, raw UTF-8 headers or unrestricted binary bodies.

DIAGRAMRead RFC section 5 ↗
RFC 6152 · § 5EXAMPLE
4 / 4
Separate body support from other capabilitiesFour separate scope comparisons assume only8BITMIME support. ASCII MAIL and headers can surround a chosen UTF8 MIME body. Raw UTF8 Subject text requires separate internationalized-header and SMTPUTF8 support. Internationalized envelope address café at example com and BODY BINARY are independent unsupported requests stopped at the sender;8BITMIME does not grant either. Finally preserving a body is distinct from authentication, encryption or malware safety;7bit MIME can carry the same information.Only 8BITMIME is advertised in these examplesPreserved and readable bodyinfer trusted / encrypted / safeNo sender authentication or encryption addedMIME encodings do not make content safe7-bit MIME can carry the same information
04
Check security separately.

A readable body preserved through transport is not authenticated, encrypted or proven safe. 7-bit MIME encodings can carry the same information, so the high-bit option is not a malware-safety gate.

Source for this drawing: RFC 6152 · Read RFC section 5 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
New to this? Start hereA quick picture + helpful background

A message needs an address.

Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.

Helpful first, if these ideas are unfamiliar

RFC 5321 SMTP RFC 2045 MIME message bodies RFC 3629 UTF-8

These pictures are analogies to help you learn. The linked RFC explains the actual rules.

Related RFCs and changes

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KEY POINT

A negotiated representation is not an identity or safety check.

  • RFC 5321 does not let 8BITMIME authorize non-ASCII envelope commands or unrestricted 8-bit material; high-bit content still needs appropriate MIME formatting and transfer encoding.
  • SMTPUTF8 under RFC 6531 and UTF-8 header syntax under RFC 6532 are separate related mechanisms. SMTPUTF8 servers must advertise 8BITMIME, but 8BITMIME alone does not imply SMTPUTF8.
  • RFC 6152 adds no message confidentiality or sender-authentication mechanism. MIME transport encodings do not make content safe or prevent malware.
DETAILS

The stopped examples assume only 8BITMIME is advertised. Raw UTF-8 headers and addresses would need separate SMTPUTF8 support; encoded ASCII header text is a different technique. BINARYMIME/CHUNKING under RFC 3030 are also distinct mechanisms, outside this guide.

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