I am facing an issue " Denied connection per minute from one ip address" why this error occur and how to resolve it? is it really harmful for my TMG Server or not??

I am facing an issue " Denied connection per minute from one ip address" why this error occur and how to resolve it? is it really harmful for my TMG Server or not??

Error Description:

The number of denied connections from the source IP address 10.0.0.X exceeded the configured limit. This may indicate that the host is infected or is attempting an attack on the Forefront TMG computer. 

electrifying