X-Originating-IP

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.^[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]

See also

• Internet privacy
• List of proxy software
• X-Forwarded-For