Domain reputation is the invisible lever behind inbox placement. Send from a trusted domain and Gmail delivers. Send from a damaged one and every email goes to spam - regardless of how good the content is. This guide covers how reputation works, how to check yours, what damages it, and how to repair it.
Reputation is not a single number from a single source. Every major mailbox provider (Gmail, Outlook, Yahoo, iCloud) runs its own scoring model. The factors below contribute to reputation across all of them, though weightings differ.
Permanent bounces (5xx errors) signal you are sending to non-existent addresses. Above 2% consistently > reputation damage. Above 5% > likely blacklisting. Hard bounces must be suppressed immediately and never retried.
Recipients hitting the spam button is the single most damaging signal. Google enforces a 0.08% threshold (1 complaint per 1,250 emails). Sustained above 0.1% triggers bulk filtering. Postmaster Tools shows this per-domain in real-time.
Opens, replies, and link clicks signal that recipients want your email. Low engagement (0% open rate on a new domain) triggers spam filter suspicion. High engagement lifts domain reputation over time, particularly with Gmail.
Sudden volume spikes from a new or low-volume domain look like spam infrastructure. Starting at 20-50 emails per day and increasing by 30-50% per week over 4-6 weeks is the standard warmup pattern that avoids triggering spam filters.
SPF, DKIM, and DMARC are table stakes. Failing authentication on any of these means your email will be rejected or spam-filtered regardless of reputation history. DMARC with p=reject prevents domain spoofing.
Being listed on Spamhaus, Barracuda, or Proofpoint blacklists results in rejection at major hosting providers. Regular monitoring with MXToolbox (blacklist check) is essential for any active sending domain.
High/Medium/Low/Bad domain reputation for Gmail, spam rate per-day, authentication pass rates, delivery errors. The only tool that shows Gmail internal reputation directly. Requires DNS verification. Check weekly if actively sending cold email.
Checks your domain and IP against 100+ blacklists simultaneously. Shows which lists you are on and links to removal request forms. Run this immediately if campaign performance suddenly drops.
Send a test email to a unique address they provide. Returns a score out of 10 covering: SPF pass, DKIM alignment, DMARC policy, content spam score (SpamAssassin), blacklist check, HTML quality. Best pre-launch check for any new sequence.
Cisco Talos reputation for your domain and sending IP. Relevant for Outlook and Office 365 deliverability since Microsoft uses Talos data. Check both your domain and all sending IPs.
Sends test emails to seed accounts at 35+ mailbox providers and shows whether they land in inbox, spam, or promotions. The most accurate picture of real inbox placement across providers. Expensive but essential for diagnosing inbox placement issues.
Do not send another email from the affected domain until you understand why you were listed. Continuing to send while on a blacklist worsens your record and delays removal.
Run MXToolbox blacklist check. Note which lists you are on. Different lists have different removal processes - Spamhaus, Barracuda, Sorbs, and Microsoft all use separate portals.
Common causes: sending to purchased lists, high bounce rates (unverified addresses), compromised account sending spam, or shared hosting with a bad neighbor. Fix the cause before requesting removal or you will be re-listed.
Verify your entire sending list. Remove all invalid addresses, hard bounces, and role accounts (info@, admin@, support@). Run a secondary verification on all catch-all and risky addresses.
Spamhaus: spamhaus.org/lookup. Barracuda: barracudacentral.org/lookups. Microsoft: sendersupport.olc.protection.outlook.com. Each has a form; some require waiting a review period (24-72 hours is typical).
After removal, restart at 20-30% of your normal sending volume for 2 weeks. Send only to your highest-engagement contacts first. Let engagement signals rebuild reputation before scaling back up.
| Setting | Recommended Value | Why It Matters |
|---|---|---|
| SPF record | v=spf1 include:[sending-provider] ~all | Tells receiving servers which IPs are allowed to send for your domain. ~all (softfail) is safer than -all while warming up. Switch to -all once stable. |
| DKIM signature | 2048-bit key, aligned with from domain | Cryptographic proof that the email was not tampered with. 2048-bit minimum. Must align with the From: domain header. |
| DMARC policy | v=DMARC1; p=quarantine; rua=mailto:[email protected] | Start with p=quarantine during setup; move to p=reject once you confirm all legitimate senders pass. rua= gets aggregate reports. |
| MX record hygiene | Only list active mail servers | Misconfigured or dangling MX records cause bounce spikes and confuse receiving servers. |
| Reverse DNS (PTR) | Must match the SMTP EHLO hostname | Missing or mismatched PTR = immediate spam flag on many servers. Essential for self-hosted sending. |
| Sending domain separation | Separate domain from main website domain | yourco.io for cold email; yourco.com for brand email. Reputation damage is isolated. Never use your primary domain for cold outreach. |
Every bounce chips away at domain reputation. Verify your list before each send - remove invalids before they reach your sending server. Start free, no credit 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
9 factors, where to check (Sender Score/Postmaster/SNDS/Talos), benchmarks, step-by-step improvement plan
IP vs domain reputation, bounce thresholds, tools to check score, recovery steps
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
DNS records, DMARC policy progression, verification commands, cold email setup
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