Effect of policy enforcement on messages identified as Forwarded

Q: What happens to messages in "Forwarders", when policy enforcement (either p=quarantine or p=reject) is put into place?

A: If messages are not shown to pass DMARC, then moving to a DMARC enforcement policy should be assumed to have the requested policy (either quarantine or reject) applied to the messages. There are some services which attempt to perform more extended analysis of traffic or maintain connecting host information and thereby use intelligence to override policy enforcement decisions, but this cannot be relied on.