Há um artigo da base de conhecimento no site da WatchGuard, mas você precisa estar conectado chegar a isso, então cito:
Question
Why are internal IP addresses blocked by Default Threat Protection?
Answer
In Fireware XTM v11.x, when the Block Port Space Probes and Block Address Space Probes check boxes are selected in the Default Packet Handling configuration, all incoming traffic on all interfaces is examined by the Firebox or XTM device.
Some internal computers, usually DNS or email servers, could be unexpectedly blocked due to IP scan, or port scan protection.
There are two options to protect against this. One option is to add those servers to the Blocked Site Exceptions list.
Another option is to increase the thresholds for port or address space probes. For more information, in "Setup > Default Threat Protection > Default Packet Handling".
Acho que você deve verificar o computador para ter certeza de que ele não possui nenhum software mal-intencionado; Não me lembro de ter visto nada a ver com os Macs que acionaram sites bloqueados, especificamente.