This plugin adds headers to the start of the message as detailed below.
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If $FIXUP_RECEIVED_HOST and $FIXUP_RECEIVED_IP are set and do not exactly match the current local host and IP, a Received: header is added to show a transition from one to the other. Then a standard Received: header is added. Finally, if $HEADER_ADD is set, its contents are added to the message headers.
The standard Received: header has the following format:
Received: from helo_domain (remotehost [remoteip] by localhost ([remoteip]) with protocol via transport; date
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