Because, believe it or not, most spammers don't bother. The main server
that I maintain (_NOT_ where I receive L-K) drops at least 20 messages a
day at RCPT TO: time using this very check. Considering that's 50% of
the spam we drop at RCPT TO: time, I'd say it's worth it.
But I do agree, the systems doing the callouts should cache the results
(I'm an Exim 4.10 user as well).
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