IP Reputation
A score or classification assigned to an IP address based on its history of sending spam, hosting malware, or other abuse.
IP reputation is the aggregate judgment that mailbox providers, firewalls, and security vendors have formed about a given IP address. It is built from observed behaviour (volume, complaint rate, bounce rate, malware hosting, brute-force attempts), DNSBL listings, sender authentication results, and proprietary signals. Senders inherit the reputation of the IP they send from, which is why shared sending pools can sink a careful sender and why warmup of new IPs is critical. Reputation is operator-specific: an IP in good standing at Gmail may be blocked at Microsoft.