E.1 Inside the Message: Headers, Body and Parts

SMTP specifies how to send mail between systems, while POP3 specifies how to retrieve mail from a system. However, neither of these specifications dictate how the message content should be structured, but only provide standards for how message content should be transferred.

While the need for content standardization may not be clear at first, this is just as important as standards for moving mail between different types of systems. Without content standards, electronic mail would be of little use. For example, content standards guarantee that the "To:", "From:", "Subject:" and "Date:" fields are consistent across different mail systems and computing platforms. Since there are a wide variety of mail products on the market, each of which have their own addressing and date formats, a platform-independent method for implementing these fields is required in order for the various mail systems to exchange mail successfully.

Consistent headers are not the only area that need to be standardized. There is also a need for standard methods of publishing message bodies and attachments. Without these standards, one mail system might send 16-bit compressed text, while another may send 32-bit self-executing binary objects for the message body. Standardized content allow each of the various systems to communicate seamlessly, whether those systems are Apple II PCs or IBM mainframes.

There are three basic components of any electronic mail message sent using SMTP:

• The message envelope. The message envelope is created by the SMTP server when a new mail message is being received. It contains information pertaining to who the mail is from and who it is for. This information is not necessarily the same as that found in the message's "To:", "From:" and other fields. For more information about the envelope, refer to section E.2 The SMTP Envelope.
  
  
• The message headers. The message headers are much like those found in paper memos. They consist of "To:" and "From:" fields, as well as the "CC:", "Date:", and other fields found in the message itself (which are not necessarily the same as the addresses used in the envelope). The headers also store tracking information, allowing you to troubleshoot delivery problems. For more information on the headers, refer to section E.3 Message Headers.
  
  
• The message body. The message body consists of everything in the message, except for the headers. The message can range from nothing at all, to a multimedia extravaganza consisting of dozens of attachments. For more information on the body, refer to section E.4 Message Body.
  

Two Internet RFCs define the message envelope and the message headers and body. RFC 821 covers the SMTP protocol that is used to transfer messages between hosts, and also defines the message envelope, which is built on-the-fly by the SMTP server during transmission. Meanwhile, RFC 822 defines the basic format that a mail message should be structured in, including how the message headers and body should be presented.

For more information about the envelope, refer to section E.2 The SMTP Envelope. For more information on the headers, refer to section E.3 Message Headers. For more information on the headers, refer to section E.3 Message Headers.

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