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 server.
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The general format of the field is: X-Originating-IP: [xxx.xxx.xxx.xxx]
Where xxx.xxx.xxx.xxx is, this would be an IP address.
In 1999 Hotmail included an X-Originating-IP email header field that shows the IP address of the sender.[1][2]