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RFC 6152 / SMTP 8-BIT MIME TRANSPORT

Preserve octets and identify the right layer

02

An octet is an eight-bit byte. The MIME body and the SMTP envelope describe different things; 8BITMIME preserves high bits rather than recoding characters.

DIAGRAMRead RFC section 3 ↗
RFC 6152 · § 3EXAMPLE
4 / 4
Preserve octets and identify the right layerFour layer and bit comparisons use a chosen UTF8 café body. ASCII MIME Content Type text plain charset utf8 and Content Transfer Encoding8bit accompany body octets63 61 66 C3 A9 0D 0A. Bit rows show C3 and A9 preserved; clearing high bits instead changes them to43 and29 and corrupts text. SMTP BODY is an envelope parameter, the MIME transfer encoding is a message header, and charset interprets characters. BODY7BIT is the alternate defined value; BODY BINARY is outside this extension.Preserve bytes; interpret the charset separatelyBODY=7BITDefined alternate for a 7-bit representationBODY=BINARYuse under 8BITMIME aloneBINARYMIME is a separate extension
04
Use the defined 7-bit alternative.

For an entirely 7-bit representation, BODY=7BIT is the alternate value of the same extension. A separate BODY=BINARY request is not authorized by 8BITMIME.

Source for this drawing: RFC 6152 · Read RFC section 3 ↗

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

“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.

KEY POINT

Transport preserves bytes; the MIME charset gives those bytes meaning.

  • The receiving server shall preserve all bits in every octet passed through DATA, and after acceptance must deliver or relay without losing those bits.
  • Content-Type charset describes character interpretation. Content-Transfer-Encoding: 8bit describes a line-oriented MIME representation, not a charset conversion.
  • BODY=7BIT is the defined alternative for 7-bit content when using this extension. BODY=BINARY is not defined by RFC 6152.
DETAILS

The chosen café text uses UTF-8 bytes 63 61 66 C3 A9 and CRLF. This charset is an example, not a requirement to use UTF-8 for every body. Header fields shown here are ASCII; raw internationalized headers need separate support.

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