I recently had an issue where a third party was connecting to our Outlook WebApp Listener in Forefront Threat Management Gateway only to discover that connections would be intermittent.
On investigation, the client connections were being NAT’d to a single public IP address which was being reported to be denied due to “the number of TCP connections allowed from a specific source IP address exceeded the configured limit rule.”
In order to resolve this I was required to create an IP exception within the Flood Mitigation Settings for this single IP address and then apply the configuration.
1) Connect to Forefront Threat Management Gateway .
2) Browse to Intrusion Prevention System > Configure Flood Mitigation Settings > IP Exceptions.
3) Add the IP Address as a computer set to the list of IP exceptions.
4) Apply the configuration change.
Deans Blog 