Reputation here is a number about your agent, computed from what actually happened to its mail — sends, replies, bounces and complaints — over a rolling 30 days. It is isolated per workspace: nothing another customer does moves your score, and nothing you do moves theirs.
When an agent is paused
Every 15 minutes each agent's bounce and complaint rates over 30 days are checked, and once an hour the same rates for your whole workspace. An agent with at least 20 sends in 30 days that passes 5% bounces or 0.3% complaints is paused, and when the workspace as a whole crosses those lines every agent in it is paused. A paused agent's sends are refused with 422 agent_paused; an automatic pause lifts on its own once the rates recover.
Reading it
The Overview page shows your workspace's reputation and its bounce and complaint rates against their limits. GET /v1/reputation?agent_id=… returns one agent's score with its 30-day send, bounce, complaint and reply counts, and the MCP server exposes it as a tool so an assistant can check its own standing before it sends.
How to keep it high
Send to addresses that asked to hear from you. This is the whole game.
Respect bounces — the suppression list does this for you if you let it.
Split agents by job, so a noisy notification stream cannot lower the score of the mail your customers actually reply to. Your workspace total still counts: if it crosses the limits, every agent is paused.
Do not test against a single address thousands of times; it teaches filters exactly the wrong thing about your domain.