GMail understands ARC ("Authenticated Relay Chain"), a technology that signs a chain of deliveries, which is supposed to allow the right party to be blamed for spam. I don't know if any of these email forwarders respect ARC signature chains, but it would obviously be to their benefit if ARC's attributions were more widely recognised.
Postscript: Neither the Improvmx nor Mailgun sites have easily found information about ARC. It's a relatively complex and not widely used technology; nonetheless, the point I made about its adoption being in their interests stands.
Cf. https://www.dmarcanalyzer.com/arc-is-here/
Postscript: Neither the Improvmx nor Mailgun sites have easily found information about ARC. It's a relatively complex and not widely used technology; nonetheless, the point I made about its adoption being in their interests stands.