multipart-encoding in emails

-nemo

history of email RFCs

Date RFC Title
Jun '73 524 A Proposed Mail Protocol
Sep '73 561 Standardizing Network Mail Headers
Apr '75 680 Message Transmission Protocol
May '77 724 Proposed Official Standard for the Format of ARPA Network Messages
Nov '77 733 STANDARD FOR THE FORMAT OF ARPA NETWORK TEXT MESSAGES(1)
Aug '82 821 SIMPLE MAIL TRANSFER PROTOCOL
Aug '82 822 STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES
Jan '86 974 MAIL ROUTING AND THE DOMAIN SYSTEM
Apr '86 983 ISO Transport Services on Top of the TCP
May '87 1006 ISO Transport Service on top of the TCP
Oct '89 1123 Requirements for Internet Hosts -- Application and Support
Dec '89 1138 Mapping between X.400(1988) / ISO 10021 and RFC 822
Mar '90 1148 Mapping between X.400(1988) / ISO 10021 and RFC 822
May '92 1327 Mapping between X.400(1988) / ISO 10021 and RFC 822
Aug '93 1495 Mapping between X.400 and RFC-822 Message Bodies
Nov '95 1869 SMTP Service Extensions
Mar '97 2111 Content-ID and Message-ID Uniform Resource Locators
Jan '98 2156 MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME
Aug '98 2387 The MIME Multipart/Related Content-type
Aug '98 2392 Content-ID and Message-ID Uniform Resource Locators
Apr '01 2821 Simple Mail Transfer Protocol
Apr '01 2822 Internet Message Format
Oct '08 5321 Simple Mail Transfer Protocol
Oct '08 5322 Internet Message Format

email protocols

  1. how to talk to an email server (SMTP)
  • TCP
  • DNS
  1. how to format a message?
  • Media types (MIME)
  • how to format dates?
  • html
  1. how to receive email?
  • IMAP
  • POP

mainline

Date RFC Title
Aug '82 821 SIMPLE MAIL TRANSFER PROTOCOL
Aug '82 822 STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES
Apr '01 2821 Simple Mail Transfer Protocol
Apr '01 2822 Internet Message Format
Oct '08 5321 Simple Mail Transfer Protocol
Oct '08 5322 Internet Message Format

how to send emails

Date RFC Title
Aug '82 821 SIMPLE MAIL TRANSFER PROTOCOL
Apr '01 2821 Simple Mail Transfer Protocol
Oct '08 5321 Simple Mail Transfer Protocol

how to write emails

Date RFC Title
Aug '82 822 STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES
Apr '01 2822 Internet Message Format
Oct '08 5322 Internet Message Format

the multipart-variation

Date RFC Title
Mar '97 2111 Content-ID and Message-ID Uniform Resource Locators
Aug '98 2392 Content-ID and Message-ID Uniform Resource Locators

what's multi-part?

POST / HTTP/1.1
[[ Less interesting headers ... ]]
Content-Type: multipart/form-data; boundary=--boundary
Content-Length: 834

--boundary
Content-Disposition: form-data; name="text1"

text default
--boundary
Content-Disposition: form-data; name="text2"


From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/alternate; boundary="never-graduate";
--never-graduate
Content-Type: Text/HTML; charset=US-ASCII
Content-Id: <html.foo1@bar.net>

This is <strong>My HTML</strong> email.

--never-graduate

Content-Id: <plaintext.foo1@bar.net>
Content-Type: text/plain

This is my plaintext email.

--never-graduate--

multi-part-linking

Date RFC Title
Mar '97 2111 Content-ID and Message-ID Uniform Resource Locators
Aug '98 2392 Content-ID and Message-ID Uniform Resource Locators

multi-part-linking

Scheme Name
cid: Content ID URL Scheme
mid: Message ID URL Scheme

definitions

content-id    = url-addr-spec
message-id    = url-addr-spec

url-addr-spec = addr-spec  ; URL encoding of RFC 822 addr-spec

cid-url       = "cid" ":" content-id
mid-url       = "mid" ":" message-id [ "/" content-id ]

plain english

  • Content and Message IDs are emails
  • CID/MID URLs are cid:$CID, mid:$MID, and mid:$MID/$CID

so how does it work?

From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/related; boundary="never-graduate";type=Text/HTML

--never-graduate
Content-Type: Text/HTML; charset=US-ASCII

to the other body part, for example through a statement such as:
<img src="cid:foo4*foo1@bar.net">

--never-graduate

Content-ID: <foo4*foo1@bar.net>
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64

R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...

--never-graduate--
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