When email is forwarded, SPF breaks — the message is now arriving from KaiMail's servers, not the original sender's. Without anything to account for this, receiving servers see a broken authentication chain.
ARC — Authenticated Received Chain — solves this. It lets each handler in the forwarding path add a signed record of what it received and what the authentication state was at that point. The final recipient can verify the full chain was legitimate.
KaiMail adds a proper ARC seal at every hop. Most basic forwarders do not do this. It is why KaiMail-forwarded mail is treated as trustworthy by Gmail, Outlook, and other major providers.
For a full technical explanation: The Chain of Custody: How ARC Keeps Forwarded Email Trustworthy