Internet Mail Messages

An Internet mail message is composed of the following elements:

Envelope

Message

For complete information on message envelopes and headers, respectively, refer to RFCs 821 and 822.

Message Envelope

The IMTA uses the contents of the envelope to make routing decisions. It does not use the content of the message. The content of the envelope is primarily defined RFC 821. It includes the originator address, the recipient(s) address(es), and envelope ID. The IMTA supports additional envelope information related to SMTP service extensions published after RFE 821 such as notary (the ability to specify requested Delivery Status Notification for each recipient, see RFE 1891) and original recipient addresses

Message

RFC 822 defines a message as headers and contents.

Message Headers

An Internet mail message starts with one or more headers. Each header is composed of a field name followed by a colon then a value. Values can be generated by the composer of a message, the mail client, and IMTAs. Headers contain the following types of information about the message:

Delivery information (for example, TO, CC, BCC, From, reply-to, in-reply-to)

Summaries of the content (for example, subject, keywords, comments).

Information that enables tracing of a message if problems occur (for example, message-ID, references, received, return-path).

Information specific to the Multipurpose Internet Mail Extensions (MIME). For more information on MIME and MIME-specific headers, refer to Message Content/MIME.

RFC 822 defines several headers. Not all defined headers need to be present in a message. In fact, only a few headers are required for any type of message.

The administrator of a mail client can construct a template of desired headers that the composer of a new, forwarded, or replied-to message fills in. The following is an example of a simple template of headers for a new message:

To:
Subject:
Date:
Cc:

The mail client can automatically generate some headers (From, reply-to, message-ID, and references).

Before the message is submitted to the IMTA, the mail client can add a date header. If the From header contains multiple email addresses or if the email address is different than the submitting mail client, then the sender header is added.

An IMTA can also add headers to a message. Each IMTA that accepts a message adds a received header to that message. The last IMTA to accept the message and to actually deliver the message to the message store adds a return-path header. The received and return-path headers provides information that enables you to trace the routing path taken by the message if a problem occurs.

Message Content/MIME

A blank line separates the headers and the content or body of the message. The content or body of the message provides the data that the originator of the message intends to transmit to the recipient.

SIMS supports Multipurpose Internet Mail Extensions (MIME). Whereas RFC 822 is limited to handling text messages of a single body part, MIME extends RFC 822 to handle multiple body parts.

Therefore, the content of a message can include text as well as images, audio, video, and binary or application-specific files. The text included in the message content has the following characteristics:

Unstructured or structured

Unlimited line length or overall length

Non-ASCII character sets, which allows non-English language text

Multiple fonts

If included, the images, audio, video, and binary or application-specific files appear as attachments.

MIME defines the following headers that can appear at the start of an Internet mail message:

MIME-version - Specifies a version number to indicate that a message format conforms to the MIME standard

Content-type - Specifies the type or subtype of data in the content or body of a message. Possible values include the following

Text - Indicates data that is principally text

Multi-part - Indicates a message consisting of multiple body parts, each having its own data type

Application - Indicates either application or binary data

Message - Indicates an encapsulated message

Image - Indicates still image (picture) data

Audio - Indicates audio or voice data

Video - Indicates video or moving image data, possibly with audio as part of the composite video data format

Content-transfer-encoding - Specifies how the data is encoded so that the data can traverse Internet Message Transfer Agents (IMTAs) outside of the SIMS email system that may have data or character set limitations

Content-ID - Specifies an ID for a message content or body

Content-description - Text that provides descriptive information about the message content or body

For complete information on MIME, refer to RFCs 1521, 2045, 2046, 2047, 2048, and 2049.