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
The general format of the field is:
X-Originating-IP: [198.51.100.1]
Origins
In 1999
Hotmail included an
X-Originating-IP email header field that shows the IP address of the sender.
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.
[ what does X-EIP mean in an email message source]
See also
