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)
RFC 1123 introduced two very convenient but easily abused features: relaying without regard to recipient (open relays) and forwarding without regard to sender. Both features have been abused to the point of unusability. Open relays have been suppressed via blacklisting. SPF stops forwarding without rewriting, but it does so on an opt-in basis. If you, as a recipient do not check SPF, then you can continue to use forwarding without rewriting the sender as before. However, if you do check SPF, and you wish to reject messages that fail SPF, then you must do one of two things to avoid rejecting legitimate mail:
1. whitelist forwarder IP addresses
2. use forwarders that rewrite the sender
Note that in either case, if your forwarders do not check SPF themselves, then you will not be able to detect forgeries for mail that they relay.