Low Deliverability: Causes, Fixes & Checklist
Diagnose why your emails land in spam. Identify the root cause and fix it in order: authentication first, warmup second, list quality third.
Fast Diagnosis
3 root causes behind 80% of inbox failures
Authentication failures are the single most common cause. If SPF, DKIM, or DMARC are misconfigured, receiving servers cannot verify your sending identity and route to spam regardless of list quality or content.
Root Causes
7 root causes: authentication to blocklists
| Root cause | How to confirm | Urgency |
|---|---|---|
| Authentication failure (SPF / DKIM / DMARC) | Run domain through Mail-Tester or GlockApps. Any failing or missing record confirms the issue. | High |
| Sending from an unwarmed domain or mailbox | Check sending domain age and warmup history. Domain under 60 days old or no warmup tool running. | High |
| Bounce rate above 3 to 5% | Check bounce reports in your cold email tool. Anything above 3% signals a list quality problem. | High |
| Domain or IP on a blocklist | Run domain and sending IP through MXToolbox or GlockApps blocklist check. | High |
| Send volume too high for account age | Compare daily send volume against warmup stage. Over 50 emails/day before 4 weeks of warmup is a common trigger. | Medium |
| Spam-triggering content or links | Run copy through Mail-Tester. Check for spam-trigger words, image-heavy HTML, or links to poor-reputation domains. | Medium |
| High spam complaint rate | Check Google Postmaster Tools. A spam rate above 0.10% suppresses inbox placement at Gmail. | High |
A domain on Spamhaus, Barracuda, or SORBS causes near-total inbox failure. Pause outbound on the affected domain, request delisting, and resolve the underlying cause before resuming.
The Fix
7-step fix: authentication first, then volume
Work through fixes in this order. Authentication is the foundation: no other fix produces lasting results until it is correct.
- Run a placement test to confirm inbox vs spam status
Send a test to a seed list using Mail-Tester (free, 3 tests/day) or GlockApps. A score below 8/10 identifies the specific issues to fix.
- Verify and correct SPF, DKIM, and DMARC records
SPF must include all authorized sending services with under 10 DNS lookups. DKIM must align with the From domain, and DMARC requires a valid rua address at minimum p=none.
- Confirm warmup is active and sized for send volume
Warmup volume must match or exceed outbound volume. Any inbox with warmup paused or never started should be removed from active sequences.
- Verify the list and suppress risky addresses
Run the entire list through Bouncer, ZeroBounce, Emailable, or NeverBounce. Remove all invalid, risky, or unknown addresses and re-verify any list older than 30 days.
- Check domain and IP blocklist status
Run your sending domain and IP through MXToolbox or GlockApps blocklist checker. Identify the listing reason, fix the cause, then submit a delisting request.
- Audit email content for spam triggers
Run your template through Mail-Tester and remove spam-trigger words, poor-reputation tracked links, and image-heavy HTML. Plain text consistently achieves higher inbox placement than rich HTML in cold outbound.
- Reduce daily send volume during recovery
Cut outbound volume 50 to 70% on affected inboxes. Scale back up only after a placement test confirms inbox delivery above 85%.
Authentication fix plus list re-verification resolves most cases within 5 to 7 days. If placement does not recover, rotate to a clean secondary domain with proper authentication.
Prevention
4 prevention gaps that cause relapses
Most recurring deliverability problems come from three gaps: no placement monitoring, no re-verification cadence, and no ongoing warmup. All three are fixable with lightweight automation.
It is the only authoritative view of how Gmail sees your sender reputation. Set it up for every sending domain and treat a spam rate above 0.10% as high urgency.
FAQ
Recovery time, domain count, and open rate signals
Fix low deliverability with the right tools
Browse verified warmup tools, inbox placement testers, and authentication setup guides.