Sender Base is a way of tracking the overall health of an IP address. It uses a broad set of attributes and metrics to gauge the health.

Good

The overall sending behaviour and activity being sent from the IP is good. Email traffic is not likely to be filtered or blocked.

Neutral

The overall sending behaviour from the IP address is within reasonable boundaries. However, email traffic may still be filtered or blocked.

Poor

The overall sending behaviour from the IP address is poor. Email traffic is likely to be filtered or blocked. 

SenderBase has now become Talos Intelligence from Cisco and is available at https://www.talosintelligence.com