Understanding Email Headers

Everyone knows how to send and receive emails but only a few understand the concept behind the process.

What is actually happening when you click on the send button?

Emails are not sent directly from your machine to the recipient's computer. Mail servers that are setup by your ISP or the email provider you are using for instance process incoming emails and transfer them to their destination.

The information are stored in email headers so that mail servers know where to transfer the mails to. This is very similar to how the local post office handles your postcards. They look on the cover to find out where the letter needs to be sent to.

Why postcards? Because all email contents are readable unless you use some form of encryption. If you do not, and the majority of Internet users does not, your emails are readable by every server and everyone listening in as they pass through on their way to the destination.

Email headers can help you a lot, especially when it comes to spam and malicious emails that you may receive in your email inbox. The concept is similar to regular letters. Just like it is possible to write any sender address and name on envelopes or postcards, it is possible to do the same for email recipient addresses.

The article "reading email headers - all about email headers" gives you an in depth overview about email headers.

Everything is explained with examples which makes it easy to follow through, even for people who lack the tech background.  It begins with an introduction that explains "where mail comes from" and details "mail protocols" thereafter. The final part consists of a list of common mail headers which is great if you want to understand a certain part of the header.

Update: The guide is divided into four chapters. Displaying email headers explains how you display email headers in popular email clients such as Outlook, Netscape or Eudora. Please note that the article has been written in a time where popular clients like Thunderbird have not been created yet.

Received headers detail the emails origin and the route it took to get to you. These information are displayed in reverse order which means that the last received header in the email is in fact the first that received the email, and the first in the listing is usually the mail server of the email provider.

The last received header details what you can find out about the first server that processed the email. This can be helpful to identify the origin of the email which in turn can be used to report an email as spam or malicious.

Other headers finally lists other common email headers that you may come upon in emails.

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