Why SPF over PTR?

[Stephane Bortzmeyer]

On Mon, Nov 29, 2004 at 12:12:27PM -0800,
 Ludwig77 <gregrjones at yahoo.com> wrote 
 a message of 10 lines which said:

> I must not be understanding SPF correctly because I don't understand
> why it is better for preventing sender domain forgeries than doing a
> simple PTR lookup.

1) As explained by Barry Margolin, many mail servers are hosted by an
external company.

2) Even in the rare case when there is a one-to-one mapping
(universities, typically), not all machines whose PTR ends in
example.edu are authorized to send mail on the behalf of
example.edu. Universities host zombies, too.