"Paul" == Paul Iadonisi <pri(_dot_)marid(_at_)iadonisi(_dot_)to> writes:
Paul> This makes no sense. MAIL FROM is most certainly at least
Paul> as available to the MUA as Sender-ID via the Return-Path:
Paul> header.
Sadly non-compliant MTAs (which fail to add Return-Path on final
delivery) are quite prevalent in the field.
Hence my comment that an MUA cannot _reliably_ determine it.
-roy