Watchguard X500, dns error

Hi,
I've a firewall Watchguard X500.
On the log i found:

01-18 15:36:28 Deny 172.16.0.10 205.234.170.215 dns/udp 1032 53 1-Trusted
0-External ProxyDeny: DNS Invalid response (DNS-01) src_ip_nat="
x.x.x.x" src_port_nat="12133" proxy_act="DNS-Outgoing"

01-18 15:36:28 Deny 205.234.170.215 "x.x.x.x" 12133/udp 53 12133 0-External
Firebox denied 165 51 (Unhandled External Packet-00)

Where "x.x.x.x" is my external ip.

I see this messages every 2-3 minutes.

172.16.0.10 is my server DNS.
205.234.170.215 is the DNS that I use.

Why it give me continuously this error and this DENY?

Thanks!
Ska