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The most common causes: (1) authentication failures — SPF, DKIM, or DMARC not configured or misaligned; (2) poor domain or IP reputation; (3) complaint rate above 0.1%; (4) blacklist listings; (5) low recipient engagement; (6) spam-trigger content patterns. Brain's Deliverability DNA scan identifies the exact cause for your specific domain in under 60 seconds. Run a free scan →

Provider divergence almost always indicates a Gmail-specific reputation issue. Gmail's spam algorithm heavily weights domain reputation and per-user engagement signals. Check Google Postmaster Tools for your domain reputation score. Brain's Radar Monitor detects provider divergence in real time, and RCA Inspector identifies the provider-specific root cause automatically.

Sudden spam routing is almost always caused by: (1) a complaint-rate spike above 0.1% from a high-volume send or bad list segment; (2) a new blacklist listing; (3) an authentication change (adding a new ESP without updating SPF); or (4) a volume spike on a new or under-warmed domain. Brain's Signal Engine detects the causative signal within minutes; RCA Inspector traces the exact cause.

Cold-email spam routing usually traces to: sending from your primary domain instead of a dedicated sending domain, an under-warmed domain, missing or misconfigured authentication, sending volumes above safe limits for your domain age, or spam-trigger content patterns. See the complete cold email deliverability playbook →

DMARC (Domain-based Message Authentication, Reporting & Conformance) is a DNS record that tells mailbox providers what to do when an email fails SPF or DKIM authentication. Since 2024, Google and Yahoo require DMARC for all bulk senders. Even for low-volume senders, a missing DMARC record significantly increases spam-routing risk. Set up DMARC at minimum p=none with rua= reporting enabled. Complete DMARC setup guide →

SPF lists which servers are authorised to send email from your domain. DKIM adds a cryptographic signature proving the message wasn't altered in transit. DMARC ties SPF and DKIM together — it specifies what mailbox providers should do when emails fail authentication, and requires that the authenticated domain aligns with your visible From: address. All three are required for reliable inbox delivery.

This is an alignment failure. DMARC requires that the domain in your SPF or DKIM result aligns with your visible From: domain. If you send from yourcompany.com but route through an ESP that signs with the ESP's own domain in DKIM, alignment fails even though DKIM itself passes. Fix: configure a custom sending domain in your ESP so DKIM signs with your domain. Brain's RCA Inspector specifically diagnoses alignment failures.

Authentication proves you are who you say you are — it does not guarantee inbox placement. The most common causes: low domain or IP reputation, shared IP with bad senders, low recipient engagement, spam-trigger content patterns, or inconsistent sending volume. Full diagnostic guide →

Brain's Domain Health module monitors your domain and sending IPs against every major blacklist in real time — alerting you within minutes of any new listing. For a free one-time check, use MXToolbox's blacklist checker. Complete blacklist removal guide →

Step 1: Identify why you were listed using Brain's RCA Inspector. Step 2: Fix the root cause completely (stop spam flow, remove bad list segments, secure compromised accounts). Step 3: Submit a removal request to the blacklist operator. Step 4: Monitor for removal and inbox-rate recovery via Brain's Radar Monitor. Never submit a removal request before fixing the root cause — you'll be relisted immediately.

Brain's Radar Monitor tracks real-time inbox placement separately for Gmail, Yahoo, Outlook, and Apple Mail. It also runs divergence detection — alerting you when your inbox rate at one provider drops while others stay stable. What is inbox placement? → · Start monitoring free →

95%+ is a strong inbox rate across all major providers. 90%+ at Gmail is considered good (Gmail is the most demanding). Overall inbox rates below 85% indicate significant deliverability problems that need diagnosis. See benchmarks by provider → · Inbox placement vs deliverability →

Inbox placement tests use seed lists — monitored email accounts across Gmail, Outlook, Yahoo, and other providers. You send to these seeds, and the tool reports where each email landed: inbox, spam, promotions, or missing. Complete testing guide →

Brain monitors all your sending domains simultaneously, detects spam-rate spikes and reputation drops in real time, tracks inbox rates per provider, and provides root-cause analysis when problems emerge. Complete cold email playbook → · How to choose a deliverability tool →

For a properly warmed domain (after 4-6 weeks of warmup), 200-500 emails per day is a safe operational volume. For higher volumes, use multiple dedicated sending domains. The critical metric is not volume but complaint rate — keep it below 0.08% at all times. Brain's Signal Engine monitors complaint rates in real time across all your sending domains.