Validity of CNAME to a PTR

[Scott Haneda]

I am looking at PTR checking on a email server.  In test, I see there  
are a few DNS setups where they CNAME their PTR records.  From a RFC  
standpoint, is this valid, I am not finding any data to claim one way  
or the other.

If it is acceptable, and I realize this is getting as bit off topic,  
is the only recourse to simply whitelist those hosts that are doing  
this?

Here is the first one I found, that tripped up my filter test (mysql  
mailing list):
$dig -x 213.136.52.31

; <<>> DiG 9.4.2-P2 <<>> -x 213.136.52.31
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28031
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;31.52.136.213.in-addr.arpa.	IN	PTR

;; ANSWER SECTION:
31.52.136.213.in-addr.arpa. 86400 IN	CNAME	31.0-25.52.136.213.in- 
addr.arpa.
31.0-25.52.136.213.in-addr.arpa. 3600 IN PTR	lists2.mysql.com.

--
Scott