What is a sender reputation?
Sender reputation is the score mailbox providers assign to your sending domain and IP address, based on bounce rates, spam complaints, engagement and how consistently you send. It determines whether your mail reaches the inbox, the spam folder, or is refused outright, and it is rebuilt slowly once damaged.
The inputs providers weigh most heavily are the ones you control before sending: hard bounce rate, complaint rate, and whether recipients open and reply. A campaign to a stale list can move reputation in a single send and take weeks of clean sending to recover.
Reputation attaches to both the domain and the IP, which is why a shared IP at a cheap ESP is a risk — you inherit the behaviour of everyone else on it. It also attaches to subdomains, which is why serious senders isolate marketing mail on a subdomain and keep transactional mail on the main domain.
Verification is the cheapest lever on reputation because bounces are the one input you can eliminate entirely before a send rather than manage afterwards.
Last reviewed 2026-09-11