Err, actually, its the opposite that they are assuming. They assume that lack of an NXDOMAIN means the domain does exist. That is an invalid assumption.
Using DNS to determine existence is a decent heuristic, but it isn't exact. That's my point, in summary. (IOW - we agree.)
One shouldn't be doing whois queries. One just wants to know if the domain of the sender can receive email, back.
I was assuming that the check was done to see if the mail could be "traced" back - from "claimed source address" to domain name to registrant. Of course, with spoofing, nothing can be trusted, but if it is legit, you should be able to check it out.
If the case is just to see if mail can go back, then yeah, there oughta be an MX.
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Edward Lewis +1-703-227-9854
ARIN Research Engineer
Sponge Bob Square Pants? I'm still trying to figure out the Macarena.