Re: ISP phishing

[Mike Leber <mleber () he net>]

On Wed, 29 Jun 2005, Tony Finch wrote:
> On Wed, 29 Jun 2005, Brad Knowles wrote:
> >     SPF is not a panacea.
> >
> >     In fact, it is pretty much totally worthless, unless you are the sole
> > owner of a given domain and you can guarantee that all mail you ever send will
> > always be routed through the machines that you own and control, and you know
> > that you don't ever forward e-mail for any of your other accounts.

See my other email in regards to this mobile user strawman argument.  
Look in the archives for the same arguments against closing open relays.

> Actually, what you have to guarantee is that you never send email to
> anyone who forwards their email elsewhere. This is impossible.

This is incorrect.

SPF is an inbound filter.

This is in the recipients control, not yours.

Assume you send email to alice () alumni miskatonic edu and Alice forwards
that email address to alice () personaldomain org.

If the inbound mail server for alumni.miskatonic.edu has SPF or MX+
enabled for alice () alumni miskatonic edu and and you pass the test and your
mail is accepted by alumni.miskatonic.edu then that is the end of your
responsibility.

If Alice then decides to forward to alice () personaldomain org and Alice
wishes to use SPF or MX+ for the mailbox alice () personaldomain org as well
then Alice would whitelist the IP of the outbound mail server for
alumni.miskatonic.edu.

You don't have control over what forwarding, filtering, or whitelisting
Alice does with her personal mailbox.

If Alice wants to forward alice () alumni miskatonic edu to
alice () personaldomain org and use SPF or MX+ with alice () personaldomain org
presumably she won't block email from her other account and she can check
if she got it right really easy by sending email to
alice () alumni miskatonic edu.

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