E-mail headers are the headers at the beginning of an e-mail that define various properties (e.g. `From` or `Reply-To`) of the e-mail.
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.
Characteristics
A single email header has some important characteristics, including perhaps the most important part of an email - this is the KEY:VALUE pairs contained in the header. Looking at the above, you can tell some of the KEY:VALUE pairs used. Here is a breakdown of the most commonly used and viewed headers, and their values:
- From: sender's name and email address (IP address here also, but hidden)
- To: recipient's name and email address
- Date: sent date/time of the email
- Subject: whatever text the sender entered in the Subject heading before sending
Routing
Email headers also provide information on the route an email takes as it is transferred from one computer to another. As mentioned earlier, mail transfer agents (MTA) facilitate email transfers. When an email is sent from one computer to another it travels through a MTA. Each time an email is sent or forwarded by the MTA, it is stamped with a date, time and recipient. This is why some emails, if they have had several destinations, may have several RECEIVED headers: there have been multiple recipients since the origination of the email. In a way it is much like the same way the post office would route a letter: every time the letter passes through a post office on its route, or if it is forwarded on, it will receive a stamp. In this case the stamp is an email header.
