Your email deliverability score determines whether your emails land in the inbox, the spam folder, or get rejected before delivery. This guide covers how scores are calculated, the 9 factors that affect them, how to check yours, and a step-by-step improvement plan.
| Tool | What it measures | Score range | Best for |
|---|---|---|---|
| Sender Score (senderscore.org) | IP-based reputation from Validity’s network | 0-100 | Checking sending IP health |
| Google Postmaster Tools | Domain + IP reputation for Gmail delivery | Low/Med/High | Gmail deliverability |
| Microsoft SNDS | IP reputation for Hotmail/Outlook delivery | Green/Yellow/Red | Microsoft 365 / Outlook deliverability |
| Cisco Talos Intelligence | IP reputation for enterprise spam filters | Good/Neutral/Poor | Enterprise mail gateway filters |
| MXToolbox Blacklist Check | 100+ DNSBL blocklist status for your IP/domain | Listed/Not listed | Checking blacklist membership |
| Mail-tester.com | Composite score: auth + content + list practices | 0-10 | Pre-campaign audit |
1. Bounce rate
Hard bounces above 2% are the fastest way to damage sender reputation. Email service providers interpret high bounces as poor list hygiene. Target: under 0.5% hard bounce rate per campaign.
2. Spam complaint rate
Gmail (via Postmaster Tools) and Microsoft track how often recipients mark your email as spam. Above 0.1% complaints significantly damages domain reputation with those ESPs. Target: under 0.05%.
3. SPF / DKIM / DMARC
Authentication is table stakes in 2026. Missing SPF means your email can be spoofed; missing DKIM means delivery fails for many enterprise mail gateways. DMARC ties them together and provides reporting. All three are mandatory.
4. List age and hygiene
Old lists have higher invalid rates. Every 90 days without re-verification, ~6% of B2B addresses become stale. Sending to a stale list without re-verification creates bounces that hurt your score.
5. Engagement rates
Gmail and Outlook track whether recipients open, reply to, or click links in your emails. Low engagement relative to send volume is a reputation signal. Target ICP tightly to maximise engagement per send.
6. Sending volume consistency
Sudden large volume spikes (from 50/day to 5,000/day overnight) trigger spam filter suspicion. Build sending volume gradually over 4-8 weeks on new domains.
7. IP address reputation
Shared IPs (used by many senders) carry the reputation of all senders on them. If another sender on your ESP’s shared IP gets blacklisted, it can affect your delivery. Dedicated IPs give you full control at the cost of slower warmup.
8. Content signals
Spam trigger words (FREE, GUARANTEED, WINNER), excessive images, missing plain text version, broken links, and misleading subject lines all contribute to spam filter scoring, though less than bounce rate and complaints.
9. Unsubscribe rate
High unsubscribe rate signals poor targeting to some ESPs. More directly, it means your ICP definition or messaging needs work. Target: under 0.2% per campaign.
Week 1 - Audit
Check Sender Score, Google Postmaster Tools, Microsoft SNDS, and MXToolbox. Identify if you’re blacklisted, what your complaint rate is, and whether SPF/DKIM/DMARC are configured correctly.
Week 1 - Authentication
Set up SPF, DKIM, and DMARC if missing. For DMARC, start with p=none for monitoring, then move to p=quarantine after 30 days of clean data. Verify with mail-tester.com.
Week 1-2 - List hygiene
Remove all hard bounces, opt-outs, and inactive contacts from prior campaigns. Run remaining list through BounceZero to identify and remove invalid, disposable, and high-risk addresses.
Week 2-4 - Re-warmup
If your domain reputation is Low/Poor, pause all campaigns. Start fresh with 50-100 emails/day to your most engaged segment. Build to full volume over 4-6 weeks.
Ongoing - Monitoring
Set up Google Postmaster Tools and Microsoft SNDS alerts. Check bounce rate and complaint rate after every campaign. Re-verify contact lists every 90 days. Never let a list sit unused for 6+ months without re-verifying.
| Sender Score | Status | Action |
|---|---|---|
| 90-100 | Excellent | Maintain: continue best practices, monitor monthly |
| 80-89 | Good | Monitor: investigate any recent bounce/complaint spikes |
| 70-79 | Caution | Investigate: check DNSBL status, review recent campaign metrics |
| 60-69 | Problem | Pause campaigns: deep audit, list clean, re-warmup |
| Below 60 | Critical | Stop all campaigns: investigate root cause, delist from DNSBL, re-warmup from scratch |
A composite measure of how trustworthy your sending domain and IP appear to email service providers. Not a single universal number - Sender Score, Google Postmaster Tools, Microsoft SNDS, and Talos each calculate it differently. Factors: bounce rate, spam complaints, authentication, list quality, engagement, sending consistency, IP reputation, and content signals.
(1) senderscore.org - IP reputation from Validity. (2) postmaster.google.com - Gmail domain/IP reputation. (3) Microsoft SNDS - Outlook/Hotmail reputation. (4) talosintelligence.com - enterprise filter IP reputation. (5) mxtoolbox.com - 100+ DNSBL blacklist check. (6) mail-tester.com - composite audit. Check all, not just one.
Sender Score: 90-100 excellent, 80-89 good, 70-79 monitor, below 70 investigate. Google Postmaster Tools: ‘High’ is target. Microsoft SNDS: Green is target. Each ESP scores independently - a good score on one doesn’t guarantee delivery to another’s users.
Bounce rate is the fastest way to damage sender reputation. BounceZero removes invalid and risky addresses before they bounce. $3/1K. 100 free credits.
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.
Deep-dive guides on how email verification and inbox placement work
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826K re-verifications: only 19% of valid addresses survive 90 days
True catch-all is 1.4% - most of what looks catch-all is unprobeable providers
info@ bounces 4.5x more than personal addresses - measured, not guessed
The 3x invalid-rate gap that vanishes when you control for domain size
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