X-Originating-IP explained

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

References

1. Web site: Q&A: Fighting Spam at MSN Hotmail . Microsoft.com . 1999-09-22 . 2012-05-28.
2. News: Declan McCullagh . The Wrong Way to Do Dirty Tricks . Wired.com . 2001-06-16. 2012-05-28.
3. https://answers.microsoft.com/en-us/windowslive/forum/email/emails-from-hotmail-no-longer-present-x/eefd1c90-7b29-4c8c-ae35-6a97d43a7585 what does X-EIP mean in an email message source