E-mail Standards

There are no desktop-specific standards for sending e-mail. There are, however, standards that describe how messages are relayed within the Internet messaging infrastructure. In the early days of Internet e-mail, the origination of messages from the desktop to the Internet messaging infrastructure was considered a local matter. There were a number of platform-specific technologies for doing that. Today, however, the same protocol is used both for submitting messages to the Internet messaging infrastructure and for when the infrastructure relays those messages.¹ That standard is called the SMTP [Simple Mail Transport Protocol]. Before considering SMTP in greater detail, we need to say something about messages with structured bodies.

Structured Messages

The history of Internet e-mail starts with only two sets of structures: the structure of an envelope for relaying messages throughout the infrastructure (we'll discuss this later on in this excerpt); and the structure of headers used within a message. Messages were assumed to be unstructured plain text. There are two underlying assumptions for such a system. First, that the message bodies contain characters used only for printing or formatting (commonly referred to as "printable ASCII"); and, second, that humans, rather than programs, are the ones processing the message when it is received.

Clearly, both of these are poor assumptions. This led to numerous systems being developed for "multimedia e-mail" throughout the pre-modern Internet. After considerable experimentation, the community standardized one particular approach, called Multipurpose Internet Message Extensions (MIME). Although there are some six or so documents that comprise the MIME standard, there are really only two key topics:

1. labeling a message body so that a program can uniquely identify the type of content contained in the message
2. encoding a message body so that if it contains nonprintable or binary characters, these can be safely transmitted via e-mail.

Let's reiterate that for clarity: MIME structures and encodes data. The structuring part is done using a Content-Type:, and the encoding part is done using a Content-Transfer-Encoding:. We now look at each.

1 In systems that aren't 100% pure Internet, there are proprietary techniques for e-mail submission. As a rule, these are highly suboptimal, since they tend to introduce the same sort of problems that one sees with messaging gateways.