X-Originating-IP is a de facto standard email header field 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.12 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
References
References
- "Q&A: Fighting Spam at MSN Hotmail". Microsoft.com. 1999-09-22. Retrieved 2012-05-28.
- Declan McCullagh (2001-06-16). "The Wrong Way to Do Dirty Tricks". Wired.com. Archived from the original on 2014-05-17. Retrieved 2012-05-28.
- what does X-EIP mean in an email message source