One philosophy would consider a mailserver having no reverse DNS entry as not being a legitimate mailserver.
This is perfectly justifiable if stop to ask "what legit server can't/won't have PTR, __ANY__ PTR?? Darn, even 100k spammers on subscriber networks have PTRs.
With the RBLs being taken out, the credentials tactic makes more sense. and no PTR is mother of all missing credentials.
bad helo (totally under control of the mail admin) is the son of a mother.
There must be some pressure on legit servers to look legit, and NOT to look like a spammer.
Len
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