When you receive an email, the header at the top of the message includes multiple pieces of data about the email. In addition to telling you who sent the email, when the email was sent, when the email was delivered, the subject of the email and other information, the header will also have an X-Mailer line. In essence, the X-Mailer line in the email header tells you what program was used to draft and send the original email "(See Reference 1)".
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How to View Header Information
Visibility of email headers vary slightly depending on which email client you use or which one the sender used. Some email programs will display headers by default, while others—such as Gmail—will hide them to offer a cleaner and simpler user experience.
Therefore, if you want to view header information—including the X-Mailer line in Gmail, you can do so by opening a specific email message, clicking the down arrow next to the Reply button, and click Show original from the drop-down menu. The raw or original copy of the email will allow you to peruse all header information included with the message.
Even after you find the header information for your emails, you may not be able to find the X-Mailer line. Continuing with the Gmail example, you will only be shown an X-Mailer line if the email in question was sent using an actual email program on the sender's computer. As webmail clients are becoming increasingly common, X-Mailer headers are actually becoming less common. However, if your sender emails you using Apple Mail, Thunderbird, Outlook or another actual email program, you should still be shown an X-Mailer.
The Uses of an X-Mailer Header
The X-Mailer header is good for satisfying curiosity about where an email message came from, but it also has practical benefits. Spam filters will sometimes look at the X-Mailer field in order to more efficiently block unwanted messages. If the header contains the name of a program used to send spam, then the spam filter can toss the message directly into the junk folder. Unfortunately, many spam senders these days are able to send messages without an X-Mailer header or to simply use popular email programs, which no spam filter is going to flag, such as Microsoft Outlook. As a result, the X-Mailer header is only partially helpful for filtering spam.