Sender reputation is the trust score mailbox providers assign to your sending domain and IP, based on authentication, engagement, complaints, and bounces. It is the single biggest factor in whether cold email reaches the inbox.
What builds and breaks it
Reputation rises with authenticated, wanted mail that earns opens and replies, and falls with spam complaints, high bounce rates, spam-trap hits, and volume spikes from a cold domain. It is tracked per domain and IP, so one bad campaign can tax everything you send afterward.
How to protect it
Authenticate with SPF, DKIM, and DMARC, warm the domain, verify lists to keep bounces low, and design first emails to earn replies. Reputation is slow to build and fast to burn, which is why deliverability is a discipline, not a setting.