All Internet electronic mail is built upon the simple text message format described in RFC 822. This format dictates that electronic mail messages consist of lines of text.

Extensions to the basic text message format provide the basis for mail with binary attachments and digital signatures and that is integrated with the X.400 mail system. Some of these extensions will be covered in later chapters, especially those that deal with Multipurpose Internet Mail Extensions (MIME). (X.400 mail gateways are not covered in this book, but URLs for the applicable RFCs are listed in Appendix A, Internet RFCs Relating to Email.)

This chapter introduces the format of simple text messages, including the concepts of message headers and bodies. The section “Headers” provides a detailed discussion of headers, their use, and restrictions.