Messaging Gateway logs errors to the MTA / mail log indicating that the DNS backend has been set to some value, usually 'ares' or 'unbound'.
2015 Aug 16 19:28:39 PDT (err) ecelerity:  setting dns backend to 'ares'
These messages are not errors and merely indicate that the DNS configuration is being set. The messages are being logged at error level but should be logged at information level and can be safely ignored.
This issue has been addressed with the SMG 10.6.3 release.