If forwarding is failing DMARC, that's on your system, not the senders. There is nothing the sender can do to help the situation. Basically DMARC needs either

If forwarding is failing DMARC, that's on your system, not the senders. There is nothing the sender can do to help the situation. Basically DMARC needs either DKIM OR SPF to pass to consider a message passing. Being no SPF record will ever include (forwarders aren't allowed to originate messages from the domain) that means DKIM has to pass. DKIM works by signing the message body and selected headers with a private key, while the public key is published to DNS. This allows a receiver to fetch the public key from DNS and validate it was really sent by the sender. It should be clear at this point that you'll be able to forward a DKIM signed message as much as you want as long as you don't modify the message when sending. If on the other hand your forwarder modifies the headers (including subject) or the body DKIM will fail. In other words, if your going to use a forwarder, make sure that forwarder will forward the message as is. You can add headers, but don't remove or modify any existing headers and don't modify the message itself.