Internet Email Standards

Within the Internet, everything is tried, some of it works, and those that survive long enough without being superceded become standards. Some elements of the Internet mail system have become standards, and some have not. In any case, the Internet is always in motion, so at any time the Internet mail system as a whole consists of many parts that are not strictly standard but are very widely used.

The hierarchy of Internet standards looks like this:

Standards

Protocols adopted as standard for the Internet by the Internet Engineering Steering Group (IESG). Standards have STD numbers, as listed in RFC 1311.

Draft standards

Protocols that are being considered by the IESG as possible standards. Substantial and widespread testing and comments are desired.

Proposed standards

Proposals that the IESG may consider for standardization. Revision of specifications is likely while testing occurs.

Experimental protocols

Protocols that should only be implemented by those participating in a coordinated experiment.

Documented Internet protocols may also be classified as Informational (published for convenience) or Historic (superceded or uninteresting).

Standards are also given a status: “required,” “recommended,” “elective,” “limited use,” or “not recommended.” A host or gateway connected to the Internet must implement required standards and should implement recommended ones. If a host or gateway offers a service covered by an elective standard, it should follow that standard.

The parts of the Internet mail system that are standard are SMTP, including some of its extensions, the format for textual messages as specified in RFC 822, and the Post Office Protocol Version 3 (POP3). Other parts of the mail system may be widely implemented but are not finalized standards. (See Table 1-1.)

SMTP forms the backbone of the Internet mail system and is the primary transmission medium of mail across the Internet. POP3 is used by intermittently connected hosts or networks to retrieve mail from a permanently connected mail host (sometimes called a maildrop). The newer IMAP protocol, currently in its fourth version, is gaining favor as a more network-centric option.

Table 1-1. Internet Standards Relating to Electronic Mail

RFC Number

STD Number

Status

Title

821

10

Recommended

Simple Mail Transfer Protocol (SMTP)

1870

10

Recommended

SMTP Service Extensions for Message Size

1869

10

Recommended

SMTP Service Extensions

822

11

Recommended

Format of Electronic Mail Messages

1049

11

Recommended

Content-type Header Field

1939

53

Elective

Post Office Protocol (POP) Version 3

The extensions to Internet mail that allow for the attachment of arbitrary (binary) files to a mail message are covered in a series of RFCs known collectively as the Multipart Internet Mail Extensions (MIME). The core MIME RFCs are draft standards. (See Table 1-2.)

Table 1-2. Draft Internet Standards Relating to Electronic Mail

RFC Number

Status

Title

2049

Elective

MIME Conformance Criteria

2047

Elective

MIME Message Header Extensions for non-ASCII

2046

Elective

MIME Media Types

2045

Elective

Multipurpose Internet Mail Extensions (MIME)

Many proposed standards relate to enhancements of MIME or SMTP. Some deal with proposed privacy enhancements to Internet mail, such as PGP/MIME (now Open PGP) and Privacy Enhanced Mail (PEM). This list changes rapidly: The Internet is always in flux.

Some of the most important proposed standards detail the creation of a new way for remote or intermittently connected users to manipulate mail at a distance. These proposals are known as the Internet Message Access Protocol (IMAP), currently in version 4 revision 1. (See Table 1-3.)

Table 1-3. Proposed Internet Standards Relating to Electronic Mail

RFC Number

Status

Title

2440

Elective

OpenPGP Message Format

2426

Elective

vCard MIME Directory Profile

2425

Elective

A MIME Content-Type for Directory Information

2244

Elective

ACAP—Application Configuration Access Protocol

2222

Elective

Simple Authentication and Security Layer

2195

Elective

IMAP/POP AUTHorize Extension for Simple Challenge/ Response

2112

Elective

MIME Multipart/Related Content-type

2088

Elective

IMAP4 Non-Synchronizing Literals

2087

Elective

IMAP4 QUOTA Extension

2086

Elective

IMAP4 ACL Extension

2077

Elective

Model Primary MIME Types

2060

Elective

Internet Message Access Protocol (IMAP) Version 4 Revision 1

2034

Elective

SMTP Enhanced Error Codes

2015

Elective

MIME Security with PGP

1985

Elective

SMTP Service Extension ETRN

1892

Elective

MIME Multipart/Report Content-type

1848

Elective

MIME Object Security Services

1847

Elective

MIME: Signed and Encrypted

1740

Elective

MIME Encapsulation of Macintosh Files

1734

Elective

POP3 AUTHentication Command

1731

Elective

IMAP4 Authentication Mechanisms

1495

Elective

MHS/RFC-822 Message Body Mapping

1494

Elective

X.400/MIME Body Equivalences

1424

Elective

PEM—Key Certification

1423

Elective

PEM—Algorithms, Modes, and Identifiers

1422

Elective

PEM—Certificate-Based Key Management

1421

Elective

PEM—Message Encryption and Authentication

Finally, the great laboratory that is the Internet is still developing and still producing experiments worthy of note. Through the experimental protocols, one can get a glimpse of the Internet’s future, although it will be certain to be a fuzzy glimpse. (See Table 1-4.)

Table 1-4. Experimental Internet Protocols Relating to Electronic Mail

RFC Number

Title

1911

Voice Profile for Internet Mail

1806

Content-Disposition Header

1641

Using Unicode with MIME

1505

Encoding Header Field for Internet Messages

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