Actually, there is an A record:
[root@ns1 named]# host openacs.org
openacs.org has address 208.184.248.88
And a PTR record:
[root@ns1 named]# host 208.184.248.88
88.248.184.208.in-addr.arpa domain name pointer 208.184.248.88.openforce.net.
But there's no A record for the hostname returned by the PTR:
[root@ns1 named]# host 208.184.248.88.openforce.net
Host 208.184.248.88.openforce.net not found: 3(NXDOMAIN)
The real problem is the hostname returned by the PTR doesn't have an A record associated with it (as you say). But that's a common situation. Heck, my own DNS has some placeholder PTR records such as this.
I agree it's a good idea to have to have a match between the A record for the hostname returned by the PTR record. But in reality, if you're doing email verification checks by making sure the PTR matches the A (as resolved by the SMTPD HELO/EHLO) you're gonna reject a lot of legit email--lots of folks have broken reverse DNS.
