X-Originating-IP

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X-Originating-IP is an email header that shows the original IP address of the user who sent the message to a mail service’s web frontend. It is not the same as X-Forwarded-For. When a web frontend acts as a proxy to the mail server, this header can reveal the sender’s real IP address. The header looks like: X-Originating-IP: [198.51.100.1]

In 1999, Hotmail started including this header to display the sender’s IP. In December 2012, Hotmail removed it and switched to X-EIP (encoded IP) to protect users’ privacy.