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X-Originating-IP
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<p>The X-Originating-IP (not to be confused with <a href="/facts/X-Forwarded-For/x26DUDKc">X-Forwarded-For</a>) <a href="/facts/Email_header_fields/6pD9GLtI">email header field</a> is a <a href="/facts/De_facto/3Du6WQKh">de facto</a> standard for identifying the originating <a href="/facts/IP_address/z9V22Sw9">IP address</a> of a client connecting to a mail service's HTTP frontend. When clients connect directly to a <a href="/facts/Mail_server/9SJG5sbb">mail server</a>, 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. </p>