Each major inbox provider (Gmail, Microsoft, Yahoo, Apple) maintains its own internal reputation model based on signals like spam complaints, bounce rate, sender authentication, list age, and recipient engagement. You cannot see these scores directly, but tools like Google Postmaster Tools and Microsoft SNDS surface enough proxy data to monitor trends.
Reputation is attached to two things: the sending IP and the sending domain. Shared IPs (the default on most platforms) average the reputation of every sender on that IP; dedicated IPs give you full control but require careful warm-up over 4–6 weeks before they can carry meaningful volume.
The fastest way to damage reputation is to send to a stale list, an imported list of unknown provenance, or to ignore unsubscribes. The fastest way to rebuild it is to sharply cut volume to your most engaged segment only, fix authentication, and grow back slowly over weeks.