Email Protection Error: 451 Exceeding Connection Limit
Posted by on 05 June 2015 04:00 PM

Messages sent to my users are being rejected with the error "451 Exceeding Connection Limit." What causes this error and how can it be resolved?


Messages are being denied. The recipient disposition has the following event:
451 Exceeding connection limit: RBLDNSD 

This error is caused by a filter which detects Snowshoe/Hailstorm types of spam attacks, where many hundreds of thousands of messages are delivered in a short period of time from a relatively unknown IP address.

The Email Protection Inbound Spam Classification window includes an option labeled "Enable spam flood prevention."  Enabling this option helps reduce the chance of receiving spam email messages from short lived spam campaigns known as Hailstorm or Snowshoe attacks.
To disable this option, follow the instructions below:

1. Log on to the Control Console
2. Select Email Protection
3. Select Policies
4. Highlight the desired inbound policy (usually Default Inbound) and click Edit
5. Click the Spam tab, this will default to the Classifications sub tab
6. Disable the checkbox for Enable spam flood prevention
7. Click Save. Please allow 20 minutes for full replication of the settings across our servers.

ERROR: This domain name does not match domain registered in the license key file (, allowed domains:,localhost, please change the product path to match the domain under Admin CP > Settings > General Settings
This product will not work properly unless untill that value is changed.

For more information please contact Kayako support at