After RFC 821 was mutilated by RFC 1123 email could be forged.
Forwarding was broken by RFC 1123 5.3.6(a) about 19 years ago. The
spammers figured it out about seven years ago. The concept of
"forwarding" as it was known before RFC 1123 5.3.6(a) does not work any
more, "forwarding" is now a part of the problem, like open relays.

Because people want to forward email and the ability to track them was
removed. People now can and do forge email. It has become a major
problem just like open relays. Something has to change. SPF put back a
way to tell if an email was forged.

--
Boyd Gerber <gerberb@zenez.com>
ZENEZ 1042 East Fort Union #135, Midvale Utah 84047