Summary
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.
Format edit
The general format of the field is:
X-Originating-IP: [198.51.100.1]
Origins edit
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]