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The X-Originating-IP (not to be confused with X-Forwarded-For) email header field is a de facto standard for identifying the originating IP address of a client connecting to a mail service's HTTP frontend. When clients connect directly to a mail server, its address is already known to the server, but web frontends act as a proxy which internally connect to the mail server. This header can therefore serve to identify the original sender address despite the frontend.
The general format of the field is:
X-Originating-IP: [198.51.100.1]
In 1999 Hotmail included an X-Originating-IP email header field that shows the IP address of the sender.[1][2] As of December 2012, Hotmail removed this header field, replacing it with X-EIP (meaning encoded IP) with the stated goal of protecting users' privacy.[3]
