What is Sender reputation?
Sender reputation is the score mailbox providers keep for a sending domain and IP address, built from how recipients and servers have responded to its past mail.
Also called: domain reputation, IP reputation.
Providers do not publish their formulas, but the inputs they describe are consistent: bounces, spam complaints, how many recipients open, reply to or delete messages, authentication results, and how steadily volume grows. A good reputation earns the inbox; a poor one earns the spam folder or outright rejection.
Reputation attaches mainly to the domain in modern filtering, which is why many cold emailers send from a secondary domain: if its reputation suffers, the main domain's everyday mail is untouched. A new domain starts with no reputation and has to earn one through warm-up.
The fastest ways to damage it are a high bounce rate from an unverified list and a high spam complaint rate from untargeted messages. Google's guidelines set a spam rate ceiling of 0.30% for bulk senders to Gmail.
Recovery is slow: lower volume, send only to engaged or freshly verified addresses, and let good signals accumulate.