Your sender reputation is the single biggest factor determining whether your emails reach the inbox, the spam folder, or are blocked entirely. This guide covers what sender reputation is, which factors affect your score, how to check it, and how to recover if your domain has been damaged.
Tied to the sending IP address. Shared IP addresses (used by ESP services like Mailchimp, SendGrid) mean another customer’s bad behaviour can affect your reputation. Dedicated IPs give you full control but require proper warmup (2-4 weeks of volume ramping). IP reputation resets faster than domain reputation - a new or warmed IP can recover more quickly.
Tied to the sending domain (the domain in your From address and DKIM signature). Domain reputation is increasingly the primary signal used by Gmail, Outlook, and Yahoo to classify your email. Switching IPs does not reset domain reputation - a damaged domain stays damaged until sending quality improves over weeks. Protecting your primary domain means sending cold email from secondary domains.
| Signal | Safe Threshold | Danger Zone | Impact |
|---|---|---|---|
| Hard bounce rate | < 1% | > 2% | High - inbox providers flag within days |
| Spam complaint rate | < 0.1% | > 0.3% | Critical - Gmail/Yahoo bulk sender rules trigger at 0.3% |
| Unsubscribe rate | < 0.5% | > 1% | Medium - signals content or targeting issues |
| Open rate | > 20% | < 5% | Medium - low engagement = low inbox priority |
| SPF / DKIM / DMARC | All pass | Any fail | High - authentication failure triggers spam filters |
| Blacklist hits | 0 | Any | Critical - immediate spam folder for affected providers |
| Sending volume ramp | Gradual | Sudden spike | High - rapid volume increase flags as spammer behaviour |
postmaster.google.com
Tracks domain reputation (Good / Fair / Bad / Low), spam rate, and authentication pass rate specifically for traffic to Gmail. Requires DNS verification. Shows reputation by day. The most important tool for cold email teams.
senderscore.org
Rates your sending IP reputation from 0 to 100. Below 70 = deliverability problems. A quick sanity check for IP health, but less relevant than domain reputation for modern inbox placement.
mxtoolbox.com/blacklists.aspx
Checks your IP and domain against 100+ blacklists simultaneously. Free. Run this whenever bounce rate spikes unexpectedly - blacklist listing is often the cause.
sendersupport.olc.protection.outlook.com/snds
Smart Network Data Services from Microsoft. Shows data rate, complaint rate, and status for your IP on Outlook/Hotmail/Live traffic. Requires IP registration.
talosintelligence.com/reputation_center
Cisco’s IP reputation database. Uses Good / Neutral / Poor classifications. Relevant for Cisco IronPort-based filters at enterprise mail gateways.
Never send cold email from your primary domain. Set up secondary sending domains instead - one per 500-1,000 emails per day of capacity. This protects your primary domain’s reputation for transactional and marketing email.
Use variations of your primary domain: get-company.io, trycompany.com, company-hq.io. Configure SPF, DKIM, and DMARC on each.
Start at 20-30 emails/day per domain. Increase by 50% every 3-5 days. Use a warmup network (Instantly, Smartlead, or a dedicated service) to build reputation with real inbox-to-inbox interactions.
Monitor Google Postmaster Tools for each domain. If reputation drops to “Bad”, pause that domain for 2-4 weeks. Rotate to fresh domains while damaged ones recover.
Email sender reputation is a score assigned by receiving mail servers and inbox providers (Gmail, Outlook, Yahoo) to your sending IP and domain. It measures historical quality: bounce rates, spam complaint rates, engagement, blacklisting history, and authentication compliance. High reputation = inbox; low reputation = spam folder or block.
Keep hard bounce rate under 2%. Under 1% is ideal. Between 2-5% is risky - inbox providers begin applying negative reputation signals. Above 5% triggers spam folder placement or temporary blocks from major providers.
Key tools: Google Postmaster Tools (domain reputation for Gmail traffic), Sender Score from Validity (IP reputation 0-100), MXToolbox Blacklist Check (100+ blacklists), Microsoft SNDS (Outlook/Hotmail traffic), and Talos Intelligence (Cisco IronPort filters). Use all five for a complete picture.
A verified list is the single highest-ROI action for sender reputation. Remove invalid emails before your first send. 100 free credits - no card required.
Ayoub built BounceZero's 5-stage validation pipeline, its dedicated BGP-announced IP infrastructure, and the Patroni HA PostgreSQL cluster behind every verification. Previously built high-volume email delivery infrastructure. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.
Provider behaviour, DNS setup, warm-up, and inbox placement
30 rules across authentication, list hygiene, content, infrastructure, and monitoring
9 factors, where to check (Sender Score/Postmaster/SNDS/Talos), benchmarks, step-by-step improvement plan
How to check, protect, and repair your sending reputation
Min age before cold email, warmup vs age, new domain setup checklist
Generate, publish, and verify DKIM - Google Workspace, M365, and custom SMTP
SPF syntax, provider includes, 10-lookup limit fix, ~all vs -all explained
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