What is Sender reputation?
A receiving provider's running score of a sending domain and IP, built from how recipients treat its mail — opens, replies, deletions, spam reports, bounces.
Every large provider keeps a reputation for each sending domain and each sending IP. It rises with engagement (opens, replies, mail moved out of spam) and falls with bounces to non-existent addresses, spam complaints and sudden volume spikes. Reputation is what warmup builds and what a bad list destroys, and it recovers on a timescale of weeks, not hours.
Google exposes domain reputation for your domains in Postmaster Tools; other providers don't publish it.