I once took a run at a general citations strategy to integrate
usage across Mail, News, and URIs. The application of that
scheme to this purpose would be to employ the following header in
reports of alleged spam:
This header can be inserted in any MUA which supports user-defined headers.
Encoding per 822 must be followed in headers.
Modulo encoding this should fit within the "phrase" syntax allowed
in the _standard_ References: header.
The names used are:
message_ID : Message-ID of the spam you received
addr_spec_of_originator : Account you have traced it to