reputation can be locally-maintained. Local reputation is not as powerful as shared reputation services, but does provide benefit in the short term.Yes, that's right. Local domain reputation is often expressed in terms of whitelists and blacklists. Without sender authentication, these are notoriously unreliable.
It nicely illustrates one of the benefits of authentication.
For example, users of anti-spam filters sometimes find their colleagues' email in the quarantine, so they add a wildcard whitelist entry for their domain. They soon discover that a significant chunk of spam will have their domain forged into the sender address. Without sender authentication, there's not a lot can be done about this.
However, with sender authentication, you can have a whitelisted domain entry that only allows the message a free pass if the authentication passes -- otherwise the normal spam filtering rules apply.
You could even impose a local policy that says if a message "from" our domain fails authentication, we'll reject it as spam, but this is probably too risky, at least in the early stages of deployment.
* - well, they claimed to be "Jim Fenton" and I assume it's that Jim, but perhaps it was a dog