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.{{cite web|url=http://www.microsoft.com/presspass/features/1999/09-22spam.mspx |title=Q&A: Fighting Spam at MSN Hotmail |publisher=Microsoft.com |date=1999-09-22 |access-date=2012-05-28}}{{cite news|author=Declan McCullagh |url=http://archive.wired.com/politics/law/news/2001/06/44567 |title=The Wrong Way to Do Dirty Tricks |publisher=Wired.com |date= 2001-06-16|access-date=2012-05-28}} 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.[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]