Sender reputation

Sender reputation is the score mailbox providers keep on you — per domain, per sending infrastructure, per behaviour pattern. It decides whether authenticated, well-formed mail actually reaches the inbox. Reputation builds slowly and burns fast: a single bad campaign can cost weeks of inbox placement.

What moves it

In rough order of impact:

1. Complaint rate — recipients clicking "mark as spam". Providers treat this as the strongest negative signal; sustained rates above ~0.1% are dangerous, 0.3% is a cliff.
2. Hard bounce rate — sending to addresses that don't exist reads as list buying or negligence.
3. Spam-trap hits — addresses that exist only to catch senders with bad list hygiene.
4. Engagement — opens, replies, moves-to-inbox versus instant deletes and ignores.
5. Volume consistency — steady, predictable volume looks legitimate; sudden spikes look compromised.

MiniMailer's two-layer model

Most email platforms pool customers on shared infrastructure, so one customer's bad day becomes everyone's deliverability problem. MiniMailer isolates reputation at two levels:

• Per-customer isolation. Your sending runs on infrastructure whose reputation is isolated from every other customer's — a noisy neighbour can never drag down your deliverability, and your good record is yours alone.
• Per-agent scoring within your account. Every send can carry an agent_id, and reputation is scored per agent on top of the per-domain view. When you run a fleet of AI agents, one misbehaving agent shows up in its own metrics instead of silently poisoning the shared domain.

The layers stack — neither replaces the other. Domain-level reputation is what mailbox providers see; agent-level reputation is how you find the culprit before they do.

How you're kept informed

• Dashboard — delivery, bounce, and complaint rates per domain, and per-agent reputation metrics.
• Webhooks — domain.reputation_warning and domain.reputation_recovered for domain-level movement; agent.reputation.damaged, agent.pool.promoted, agent.pool.demoted, and agent.pool.recovered for agent-level events. Wire these into your alerting — reputation problems compound while nobody is looking.
• Automatic suppression — hard bounces, complaints, and unsubscribes are suppressed immediately, so the addresses that just hurt you can't hurt you again.

Protecting it

• Double opt-in for marketing lists. The confirmation click is cheap insurance against typos, bots, and spam traps.
• Give every agent its own agent_id. Attribution is free when things are fine and priceless when they aren't.
• React to warnings. A reputation_warning webhook is the moment to pause the offending traffic and inspect it — not after the campaign finishes.
• Retire the unengaged. Recipients who haven't opened anything in months are pure downside; suppress or win them back deliberately.
• Keep transactional and marketing on separate domains so a marketing misstep never delays a password reset.