Tweak to make SPF test look at "From" header?

[HostaHost]

I'm curious if there are any changes that can be made to the SPF filter, or spam assassin, to have the "From" header of the message investigated for SPF pass/fail in addition to the Return-Path? We have customers with explicit (-all) spf records still seeing forged emails coming in claiming to be from their domain because the sender used their own return-path but forged the From header.

 

[IgorG]

Actually SPF takes all parameters from SMTP session. SPF test works before the From or Reply-TO received. It takes MAIL FROM: from SMTP session, makes DNS query to the domain, and checks IP address came from mail.
Can you show some kind of samples?