Following the update to Messaging Gateway (SMG) 10.6, a large number of DNS resolution failure messages appear in the maillog file.
2015 Dec 2 12:03:39 PST (notice) ecelerity:  DNS resolution failure 7c1767b30512b6003fd3c2e618a86522.smg.ultra.brightmail.com: 4
The DNS queries performed by the "ultra" URL filtering generates a NOTICE level message when the hostname queried does not exist in the DNS or the DNS query times out.
This issue has been addressed with the Messaging Gateway 10.6.2 release.
Please see the Messaging Gateway Administration Guide for detailed instructions on how to update the SMG software.