Checking HELO does. Checking rDNS, furthermore, discriminates against
small companies who do not own a class C IP range, and no competent
ISP is available other than via dialup (too slow) and T1 (too expensive).
As a small ISP, I watch a lot of spam, and I have yet to see a false positive
on the *existence* of rDNS. It may not match, but it likely exists, as
most ISPs put up at least *some* sort of rDNS for every IP. The
only ones who seem not to do this are in "spammer-land" (Russia, Thailand, etc)
and IPs that are hijacked.
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