It is time for some notes on Internet mail system philosophy.

Internet text messages have been around a long time. In fact, they predate the Internet as we know it today. The standard for Internet text messages is RFC 822, properly titled “Standard for the Format of ARPA Internet Text Messages.” At the time this RFC was last updated (circa 1982), the Internet was still a project of the U.S. Advanced Research Projects Agency (ARPA). The ARPANet, or ARPA Internet, often passed messages between disparate types of networks, all of which were undergoing continual development. Many mail systems existed, each with its own message format and mail handling protocols.

RFC 822 was written specifically to provide a format that transcended specific network implementations and that had the capability to represent an arbitrary amount of textual data. Note the term “textual.” X.400 and other mail systems can readily gateway to the Internet mail system because of this high-level definition, given the extensions necessary to represent all data as text. That is the job of MIME.

If an email message is sent from a different mail system (e.g., X.400), it passes through a gateway that converts its format to the Internet mail standard. The message can transit the Internet in the standard format and be delivered to any mail system on the Internet that supports the standard. Alternately, it can pass through another gateway and be translated into another local format. The intention is that any mail messages transiting the Internet be of the standard format. Also, any mail transiting a gateway should be translated out of any incoming local format into the standard, then translated into any outgoing format. The standard is the “glue.”

Just as mail transfer agents (servers) may translate between the standard and local formats, mail user agents may do the same. Mail should be both sent and received from an MUA in the standard format, but it may be held, stored, displayed, etc. in any local format that the MUA author desires.

Some local formats have become common, such as mailbox formats. Common mailbox formats for various platforms are discussed in Chapter 7, Mailbox Formats.