Programming Internet Email

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Within the Internet, everything is tried, some of it works, and those that survive long enough without being superceded become standards.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Documented Internet protocols may also be classified as Informational (published for convenience) or Historic (superceded or uninteresting).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