What is sender reputation and how do you build it?

Sender reputation is the cumulative trust score every major mail provider tracks for your sending domain and IP. It is not one number. Gmail, Microsoft, Yahoo, AOL, and the others each maintain their own internal scoring. The inputs are largely the same across all of them: authentication (SPF, DKIM, DMARC), bounce rate, complaint rate, engagement rate, sending consistency, and history.

What goes into the score

• Authentication setup. SPF + DKIM + DMARC all passing on every outbound message. Missing or failing any of the three is a deduction.
• Bounce rate. What percentage of your sends bounce. Above 2-3% is concerning; above 5% is a red flag; above 10% triggers active demotion.
• Complaint rate. Percentage of recipients who hit "Mark as spam". Target below 0.1%; anything over 0.3% causes serious damage.
• Engagement rate. Opens, clicks, replies, forwards. Higher engagement = higher reputation = better inboxing of future sends.
• Spam-trap hits. Sending to known traps is one of the fastest paths to a destroyed reputation. Even one hit registers.
• Sending consistency. Sending volume that fluctuates wildly looks suspicious. Steady, predictable volume looks legitimate.

Why verification is foundational

Bounce rate is the single biggest input you can control directly. Verifying your list before you send caps bounce rate at whatever percentage of your safe-marked addresses turn out to be inactive between verification and send (typically 1–2% on a fresh verification). Without verification, bounce rate on a stale list can easily exceed 10%, which destroys reputation faster than any other single factor.

See how to reduce email bounce rate for the full mitigation playbook.

Building reputation on a new sending domain

New domains have no reputation. ISPs treat them with suspicion until they prove themselves. The right approach:

1. Set up SPF, DKIM, and DMARC before sending anything.
2. Verify any list you intend to send to.
3. Start with low volume and your most engaged recipients (people who have explicitly opted in recently).
4. Ramp gradually over 4–8 weeks (see domain warm-up).
5. Watch bounce + complaint rates daily and cut sending if either rises.

Reputation built deliberately is durable. Reputation built by blasting a cold list is destroyed before the next campaign.

Repairing damaged reputation

Once your reputation tanks, the only fix is time and consistent good behavior:

• Stop sending to your existing list immediately.
• Re-verify everything.
• Identify and remove all invalid, disposable, spamtrap, and disabled addresses.
• Resume sending only to your most engaged subset (opened or clicked something in the last 30 days).
• Treat the first 4 weeks like a fresh domain warm-up.
• Authentication must be perfect throughout.