IP and domain monitoring have several different use cases:
For blacklist monitoring, we monitor both IPs and domains in your profile against major blacklists. For tracking spam traps, your spam trap hits are organized by both the IP address and the sending domains from which they originated. Lastly, for tracking phishing attempts, we detect phishing attempts based on the sending domain.
It’s important to have visibility into reputation issues from both perspectives, especially when you’re working in a shared IP pool. While both IPs and domains are not required, they certainly provide much better visibility into what’s happening in your mail stream.