SPF "breaks" email forwarding. SRS (Sender Rewriting Scheme) is a way to fix it. It provides a standard way for forwarding MTAs (Mail Transfer Agents) to rewrite the sender address.
SRS rewrites the envelope sender address of an email message in view of remailing it. This allows email to be forwarded without breaking the Sender Policy Framework (SPF) validation at the final destination.
Learn more on WikipediaSPFinfo.org cannot be held responsible for SRS implementations described in the external links above.