For SPF, the "all" keyword is only reached if processing the previous
policy rules did not result in a positive answer, which means
"interpret this a sign that the email is likely not spam, but use the
other filtering mechanisms before taking a decision" (it's a "+1").
At that point:
"?all" means "do not interpret this as a sign that the email is likely
spam, please use the other filtering mechanism to take a decision
instead" (it's a "+0"),
"~all" means "interpret this a sign that the email is likely spam, but
use the other filtering mechanisms before taking a decision" (it's a
"-1"),
"-all" means "interpret this a sign that the email is certainly spam,
do not use any other filtering mechanisms to take a decision" (it's a
"-infinity").
That is not how the SPF specification describes SPF testing and is not
how any widely used implementation of SPF checking actually works.
A SPF check as specified and as widely implemented ENDS when any
mechanism specified in the record is matched.
This is exactly what I wrote. With capital letters to help you to parse
the sentence: "For SPF, the "all" keyword IS ONLY REACHED IF processing
the previous policy rules DID NOT result in a positive answer".
Gregory
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