SPF "breaks" email forwarding. SRS is a way to fix it. SRS is a simple way for forwarding MTAs to rewrite the sender address.

For a mail transfer agent (MTA), the Sender Rewriting Scheme (SRS) is a scheme for rewriting the envelope sender address of an email message, in view of remailing it. In this context, remailing is a kind of email forwarding. SRS was devised in order to forward email without breaking the Sender Policy Framework (SPF)