The forwarding problem
Forwarding changes the path a message takes. A receiver may see the forwarder’s IP rather than the original sender; content transformations can also affect signatures. Read the SPF and DKIM results in that context.
What ARC adds
The Authenticated Received Chain records authentication assessments across participating intermediaries using a chain of signed sets. A receiver can consider this history when assessing a forwarded message. A valid chain does not itself require delivery or establish that every intermediary is trustworthy; receiver policy still applies. [IETF RFC 8617: ARC]
Our investigation approach
Record the original sender, every known intermediary and the final recipient. Compare messages before and after the transformation where authorised copies exist. Identify who added a footer, rewrote a header or relayed the message. Then test the actual route after an approved correction.
Do not solve a forwarding issue with a blanket bypass for the forwarding domain. Ask what identity and history the receiving platform actually evaluates. Start with Email Resolution when a live route has changed behaviour.
Primary sources
Provider requirements and product capabilities can change. Check the linked source before a production change.