reject_unknown_reverse_client_hostname RFC basis?

[Blake Hudson]

I was under the impression that hosts using SMTP were required to have a 
valid reverse DNS entry. The reject_unknown_reverse_client_hostname 
restriction would then enforce this requirement. However, I can't find a 
basis for this in RFC 821, 2821, or 5321. The closest I can find is 
helo/ehlo requirements. Is there, in fact, any requirement for sending 
hosts to have a valid rDNS entry?

Thanks,
--Blake