What Is a X-Mailer Header?
Email headers provide additional information about messages you receive. The originating program provides information such as the sender and date and time and in-depth properties such as the x-mailer, which can actually tell you which program your sender used to craft her email. If your sender's email program uses this format, you can view it in your own email client once you receive the message.
"X-mailer" is a line in the header of an email message that shows which software the sender used to send the message. When applicable, this line will also show the version of the software. For example, the x-mailer may state that you sent a message with Mozilla Thunderbird 4.61.
The x-mailer line is just one line in a message header. The header includes other information, which can vary depending upon the sending program, that describes the message. By default, many email programs and websites do not show all the available header information. You may have to specify that your program shows or expands headers. In Mozilla's Thunderbird, for example, the View menu allows you to show "All" header information or a condensed "Normal" version.
Your mail program may show the "user-agent" line rather than the x-mailer in your header. This line also includes information about the originating program and version as well as the sender's operating system. Note that when a user sends a message from a browser-based email service, some header information will be included but it will not contain the x-mailer or user agent line.
X-mailer data is inserted automatically into messages by your email program. You have no control over this or other header data that your program may include with the message. However, spammers may attempt to include false header information in a message to bypass security measures. Although every email client uses a slightly different header format, each individual program uses the same format. Thus, if the user agent or x-mailer indicates that a program such as Apple Mail is the origin but the header format isn't what Apple Mail uses, the header is forged.