The "exists" mechanism is useful for all kinds of things.
Altavista is using it to profile forgeries.
Other people can use it to create special exemptions for certain users:
for example, you can say that user X is specially allowed to send mail
from network Y.
exists:%{i}.%{l}._spf.domain.com
and in dns, have: *.210.125.24.mengwong._spf.domain.com A 127.0.0.2
But it may be costly to do the lookup each time. How often are you
going to see user X sending from network Y? If a spammer decides to
forge your domain, they've gone and DOSed your DNS server.
So it would be nice if you could hardcode that relationship into the
main SPF record, which will be cached:
if:%{l}=mengwong:ip4:208.210.125.0/24
Pretty cool, eh?
What do y'all think? Would anyone actually use that?
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