It’s 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.

While it’s a non-standard header (see X-Header), it’s widely used and required more and more often: every major ESP and sender implements it and, more importantly, modern anti-spams expect to see this header to better identify the source of the mail (for example, it’s listed as Best Practice by VadeSecure).

It’s 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.

While it’s a non-standard header (see X-Header), it’s widely used and required more and more often: every major ESP and sender implements it and, more importantly, modern anti-spams expect to see this header to better identify the source of the mail (for example, it’s listed as Best Practice by VadeSecure).