I am using Spamassassin 2.63. I am wondering can Spamassassin determine that the last hop of the mail relay is a legitimate DSL /Cable modem mail server?
No. AFAIK there's no such thing as known listing of "legitimate" mailservers for dsl/cable services for SA to use.
The closest you can get is using SPF (supported in the upcoming SA 3.0), but that's up to the cable domain to create records for, and most don't.
SA can however determine if it's an illegitimate cable/dsl relay by using the dynamic-IP RBLs carefully (it does this already). No mailserver should *ever* be on a dynamic address, just because of the problems with having mail routing going to a constantly-changing IP address. In theory users with business-class static-address services shouldn't be in Dynablock.
