Mail Headers

Each mail message has a set of headers associated with it. These are lines describing things like the sender and intended recipient of the mail, record lines for each computer the mail passed through, the date and the subject.

The remainder of the mail, the useful content written by the sender, is known as the body.

Some of the header lines are written by the sender’s mail client software. Lines may be added or subtracted by systems the mailpasses through in transit. For a full discussion on interpreting headers see the external links listed here.

When mail is in transit between computers using the SMTP protocol the headers are seperated from the body by a blank line; individual header lines are not so seperated. The structure as a whole is defined by standard RFC2822 (external). When a computer stores a mail in a single file the same convention is often followed; also mail clients which do not otherwise distinguish headers from body maintain the blank line in their output.

Most mail client software only shows a subset of the header lines of a mail but can be configured to show them all. A few cannot.