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RFC 6152 / START HERE

8-bit MIME transport, before the details

SMTP is Simple Mail Transfer Protocol; MIME is Multipurpose Internet Mail Extensions. 8BITMIME names this negotiated body-transport extension.

Mail bodies may contain bytes with their highest bit set, such as the UTF-8 bytes for café. This SMTP extension lets a sending client ask a receiving server to preserve those bytes through a line-oriented MIME transfer.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Ask this hop

  2. Preserve bytes

  3. Convert or stop

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

Ordinary SMTP transport is 7-bit. Negotiating 8BITMIME avoids unnecessary body recoding on a capable hop, while requiring a relay to preserve the information when a later hop needs 7-bit MIME instead.

A familiar example

A fictional relay carries a UTF-8 body unchanged to a capable server. If the next server lacks 8BITMIME, it can choose a lossless MIME encoding or a permanent delivery failure; it cannot silently discard high bits.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

SMTPRFC 5321 · beginner overview ↗MIME message bodiesRFC 2045 · beginner overview ↗UTF-8RFC 3629 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Negotiate before sending an 8-bit bodySMTP transfers mail between a sending client and a receiving server. EHLO discovers whether this particular connection supports 8BITMIME.
  2. Preserve octets and identify the right layerAn octet is an eight-bit byte. The MIME body and the SMTP envelope describe different things; 8BITMIME preserves high bits rather than recoding characters.
  3. Keep DATA dots and line limits8BITMIME retains SMTP line structure and dot transparency. It is not unrestricted binary transport.
  4. Relay through a server without 8BITMIMEA relay that accepted an 8-bit body still has to preserve its information on the next hop. A different server may not advertise the same capability.
  5. Separate body support from other capabilities8BITMIME is a body-transport extension. It does not by itself authorize internationalized envelope addresses, raw UTF-8 headers or unrestricted binary bodies.
Start with chapter one →

This introduction explains RFC 6152 (2011). Read the original RFC ↗