Forwarding creates a basic SPF problem: the final receiver sees the forwarder’s IP but may evaluate the original envelope sender’s SPF policy.
Sender Rewriting Scheme changes the envelope sender into a domain controlled by the forwarder while encoding enough information to route bounces appropriately.
What SRS fixes
SRS addresses the SPF identity problem created by forwarding.
It does not repair a DKIM signature that was broken by content modification, and it does not guarantee DMARC success. DMARC still needs an aligned authentication path.