Sender reputation is the trust an inbox provider places in whoever sends an email. It decides, more than anything in the email itself, whether a message lands in the inbox or in spam. Providers keep it for the sending domain and the sending IP address.
What builds or damages it
- Engagement: opens, replies and emails moved out of spam help. Deleting without reading doesn't.
- Spam complaints: the strongest negative signal. Keep them under 0.1%.
- Bounces: many emails to addresses that don't exist suggest a bought or guessed list.
- Spam traps and blocklists: hitting a trap or landing on a blocklist hurts immediately.
- Volume patterns: sudden jumps from a new domain look like spam.
Building it for cold email
Warm new mailboxes up for about two weeks, ramp volume slowly, verify every list, write to the right people and keep each mailbox to a modest daily number. There's no single public score: providers keep theirs private, so watch inbox placement, replies, bounces and complaints instead.