That sounds authoritative enough! :-)
69.64.127.243 doesn't return a result for a PTR query.
SOA on isheriff.com says ns1.p20.dynect.net is authoritative for the zone.
Query ns1.p20.dynect.net for 'A' record for smtp01-in.isheriff.com and it reports "can't find smtp01-in.isheriff.com".
I'd say there are a couple of issues with the DNS records for isheriff.com...
- Missing records from the authoritative zone owner
- Rogue A records associated with a hostname
But it's not a DNSStuff.com issue.
DNSStuff is merely reporting what is out there in the DNS Wild West.