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Email format: header and body

Last Edit: 10/01/12

The standard format for an e-mail is laid down in the Request for Comments document 'RFC 5322', and is referred to as the Internet Message Format (IMF). Simple put, the format separates an e-mail into two components:

  1. Header
  2. Body

MIME is an important supplement to 'RFC 5322'. Which allows things such as non-textual attachments in e-mail bodies and multipart messages. SMTP does not require MIME to transfer an e-mail, nor POP to receive an e-mail. In essence, all e-mail's now come MIME formatted, due to the extensive non-textural features it provides.

Header

The header summerises the sender and receiver of the e-mail. Each e-mail will only contain one header. A header has many field segments within it. Below are some of the fields found within the header,

Primary fields

Secondary fields

MIME fields

For a full list of fields.

Body

After the content encoding and type are specified, the message body is where the content is provided.

The SMTP protocol transfers e-mail's from one computer to another, but is only capable of supporting 7-bit ASCII characters. This is of little use if you wish to write e-mail's in languages other than the Latin alphabet, or attach files to an e-mail. As highlighted above, the MIME header allow e-mail's to provide MIME content.

MIME is an Internet standard which allows e-mail to provide additional character sets (not just ASCII), binary attachments, multiple body parts and header fields not in ASCII. If an e-mail is formatted in HTML, a plain text version is usually sent as well, incase the receiving e-mail server does not support MIME extensions.