Internet mail messages have a standard format, which if followed, will allow many different programs to read and create messages which will be interpretable everywhere. Some mail programs, such as Elm and Pine, do not show most of the header lines in the normal message viewing mode, so that they do not confuse the user. Message headers are most important to UAs, since they read them to tell you who a message is from, its subject, etc. User Agents are programs like Pine and Elm, which provide an easy to use interface to compose mail messages. It expects that any other desired headers will be prepended to the message before sendmail receives it.
Headers can say just about anything, as long as they are in the correct format.
If headers are included in a message, they must be at the top of the message, start at the beginning of a line, and must have no blank lines between them. Among them are notes, surveys, measurements, ideas, techniques, and observations, along with proposed and accepted internet protocol standards. RFCs can be obtained from various locations and with various methods over the Internet. The following table lists RFCs which are relevant to mail.