> The result is, if they can generate sufficient volume and IP entropy,
> that some [spam] will get through due to query timeout, which has the
> same effect to the MTA as "not listed".

That depends on how DNSBLs are implemented.  My own SMTP listener, for
example, is capable of turning DNSBL query timeouts into 4xx soft
failures at the SMTP level.  (In some cases, it does so; in others, it
doesn't.  I really should go through and make sure each DNSL lookup
does what I now think of as the right thing with soft DNS failures.)

(Also, "SPAM" is a trademark of Hormel.  They've been very nice about
our (FSVO "our") use of "spam", but it behooves us to be reciprocally
nice by not using "SPAM" for anything other than their product, even
when - as here - we are not actually obliged to by law.  That's why I
replaced "SPAM" with "[spam]" in the quote above.)

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