Email Header [INFO]
Posted by on 16 January 2014 10:47 AM
In an e-mail, the body (content text) is always preceded by header lines that identify particular routing information of the message,
including the sender, recipient, date and subject.
Some headers are mandatory, such as the FROM, TO and DATE headers.
Others are optional, but very commonly used, such as SUBJECT and CC. Other headers include the sending time stamps and the receiving time stamps of all mail transfer agents that have received and sent the message.
In other words, any time a message is transferred from one user to another (i.e. when it is sent or forwarded), the message is date/time stamped by a mail transfer agent (MTA) - a computer program or software agent that facilitates the transfer of email message from one computer to another.
This date/time stamp, like FROM, TO, and SUBJECT, becomes one of the many headers that precede the body of an email.
For more information, you may refer following Wikipedia:
Google KB about Email Header: