Over on ietf-smtp, https://datatracker.ietf.org/doc/draft-kucherawy-received-state/ was developed late last year. This is an optional tweak to email Received fields allowing annotation of entry into special states of mail handling, such as quarantines or hold-for-moderation MLM actions that would help to explain large gaps in timestamp sequences.
I'm looking for a wider audience of reviewers and (hopefully) supporters. If this sort of thing seems like a good (or terrible) idea, please do follow up and comment on one of these IETF lists:
apps-discuss: https://www.ietf.org/mailman/listinfo/apps-discuss ietf-822: http://www.imc.org/ietf-822/ ietf-smtp: http://www.imc.org/ietf-smtp/index.html
How much and what kind of support is evident will guide the choice of what its next steps are (if any).