Top Email Deliverability Tools for Inbox Placement and Troubleshooting
Compare the best free and paid email deliverability tools for inbox placement testing, authentication checks, and blacklist monitoring.
Your emails either reach the inbox or they don't. When deliverability breaks — bounced invoices, missing password resets, marketing campaigns landing in spam — the cost compounds fast. The right tooling catches problems before your recipients notice.
This guide compares the most practical email deliverability tools available today, organized by what they actually test, what they cost, and which workflow fits your team. Whether you run a five-domain SaaS or manage hundreds of client domains at an agency, you'll find a concrete stack recommendation and a step-by-step testing workflow you can run this week.
How email deliverability is measured
Every deliverability tool reports some combination of these metrics. Understanding what each one actually tells you prevents misreading results.
Inbox placement rate is the percentage of sent emails that land in the primary inbox (not spam, not promotions, not lost). Measuring it requires seed-list testing — sending to known test addresses across Gmail, Outlook, Yahoo, and other providers, then checking where each message landed. An inbox placement rate below 85% across major providers signals a problem worth investigating.
Bounce rate splits into hard bounces (permanent — bad address, non-existent domain) and soft bounces (temporary — full mailbox, server timeout). Hard bounce rates above 2% suggest list hygiene issues. Tools that check SMTP connectivity help you spot server-side problems before they inflate your bounce numbers.
Spam complaint rate measures how often recipients mark your mail as spam. Google's threshold is 0.3% — cross it and your domain reputation drops. Most deliverability tools don't measure this directly; you'll need Google Postmaster Tools or your ESP's reporting for complaint data.
Authentication pass rate tracks whether your SPF, DKIM, and DMARC records are correctly configured and whether receiving servers validate them. A missing or misconfigured record doesn't always cause immediate delivery failures, but it weakens your domain's trust signal over time. This is where continuous monitoring catches drift that one-off tests miss.
Criteria for choosing an email deliverability tool
Not every team needs the same tool. Here's what to weigh:
| Criterion | Why it matters | Who needs it most |
|---|---|---|
| Seed-list testing | Only way to measure actual inbox placement across providers | Marketing teams, agencies |
| Authentication checks (SPF/DKIM/DMARC) | Catches misconfiguration before it causes failures | Every sender — non-negotiable |
| Blacklist monitoring | Early warning when your IP or domain gets listed | High-volume senders, shared IP users |
| Continuous monitoring | Detects drift and regressions between manual tests | Ops teams, multi-domain portfolios |
| API access | Enables automation, CI/CD integration, custom dashboards | Developers, platform teams |
| Regional seed coverage | Tests inbox placement across geographic providers | International senders |
| Pricing model | Per-test, per-domain, or flat monthly — pick what fits your volume | Budget-sensitive teams |
Small businesses should start with free authentication checks and Google Postmaster Tools, then add paid seed-list testing if deliverability problems persist. Agencies and enterprises managing multiple domains need continuous monitoring with API access and alerting — manual spot-checks don't scale.
Best free email deliverability tools
These tools cost nothing (or have generous free tiers) and cover essential checks.
Mail Tester (mail-tester.com)
Send a test email to a disposable address and get a 0–10 score covering SPF, DKIM, DMARC, content analysis, and blacklist status. Limited to 3 free tests per day. No API, no continuous monitoring. Best for: quick one-off checks before a campaign launch.
MXToolbox
The Swiss Army knife for DNS and email diagnostics. Free lookups for MX records, SPF/DKIM/DMARC validation, blacklist checks (across 100+ lists), and SMTP diagnostics. The free tier covers manual lookups; paid plans add monitoring and alerts. Best for: troubleshooting a specific delivery failure.
Google Postmaster Tools
Shows your domain's reputation, spam rate, authentication rates, and delivery errors — but only for mail sent to Gmail. Requires DNS verification. API available for programmatic access to metrics. Free and essential for any team sending volume to Gmail addresses.
Mailgun Email Verification (Sinch)
Primarily an email validation API (checks if addresses are deliverable before you send), but the free tier includes 100 validations per month. Useful for list hygiene. Best for: developers building signup flows who want to reject bad addresses at the door.
DomainCare Free Domain Check
Run a free domain health check without creating an account. Validates SPF, DKIM (15 common selectors), DMARC, BIMI, MX records, blacklist status across 49 sources, plus SSL, DNS, and uptime in one scan. Best for: a quick, comprehensive domain health snapshot before diving into deliverability-specific tools.
Paid tools: features worth paying for
Free tools handle spot-checks. Paid tools handle ongoing operations.
GlockApps
Seed-list inbox placement testing across Gmail, Outlook, Yahoo, AOL, and regional providers. Send your email to their seed list and see exactly which providers put it in inbox, spam, or didn't deliver it at all. Also includes DMARC analytics, blacklist monitoring, and bounce analysis. Plans start around $85/month (Essential), with discounts on annual billing. API available on paid plans. Best for: marketing teams and agencies that need provider-by-provider inbox placement data.
Mailtrap
Started as a sandbox for testing transactional emails in development. Now includes a production sending API with built-in deliverability analytics, auto IP warmup, and content analysis. Free tier includes 1,000 emails/month for sending and 100 test emails/month for the sandbox. API-first. Best for: developer teams shipping transactional email who want staging and production sending in one platform.
Folderly
AI-driven deliverability platform that combines inbox placement monitoring with email warm-up and spam trigger detection. Placement testing covers Gmail, Outlook, Google Workspace, and Microsoft 365 (no Yahoo or other providers). Pricing is per-mailbox with annual commitment (from $56/mailbox/month at volume). Best for: outbound sales teams running cold email who need warmup and placement monitoring in one tool.
Validity Everest (formerly 250ok)
Enterprise-grade deliverability suite: seed-list testing with panels across 100+ global providers, inbox placement tracking over time, reputation monitoring, DMARC reporting, and design rendering. Pricing is custom (typically $500+/month). API available. Best for: enterprises and ESPs who need comprehensive global inbox placement data and historical trends.
MailerCheck
Email list verification plus deliverability testing. Checks SPF/DKIM/DMARC, runs inbox placement tests, and scores email content for spam triggers. Pay-per-check pricing (from $10 for 1,000 verifications). Best for: teams that need list cleaning and deliverability checks in one tool.
Postmark
Transactional email service with strong built-in deliverability tooling: dedicated IP reputation management, DMARC weekly digests, and a free DMARC monitoring tool. The delivery service itself consistently ranks high in independent deliverability benchmarks. Best for: teams sending transactional email who want deliverability baked into the sending infrastructure.
Side-by-side comparison
| Tool | Free tier | Seed-list testing | Auth checks | Blacklist monitoring | API | Pricing model |
|---|---|---|---|---|---|---|
| Mail Tester | 3 tests/day | No | Yes | Yes (limited) | No | Free / donations |
| MXToolbox | Manual lookups | No | Yes | Yes (100+ lists) | Paid plans | Free / from $129/mo |
| Google Postmaster | Full (Gmail only) | No | Yes (Gmail) | No | Yes | Free |
| Mailgun | 100 validations/mo | No | No | No | Yes | Pay per validation |
| DomainCare | Free check tool | No | Yes (SPF/DKIM/DMARC/BIMI) | Yes (49 sources) | Yes | From $0 (free plan) |
| GlockApps | 2 tests on signup | Yes | Yes | Yes | Yes (paid) | From $85/mo |
| Mailtrap | 1K emails/mo | No | Yes | Limited | Yes | From $15/mo |
| Folderly | No | Yes (4 ESPs) | Yes | Yes | No | From $56/mailbox/mo |
| Everest | No | Yes (100+ panels) | Yes | Yes | Yes | Custom ($500+/mo) |
| MailerCheck | Limited | Yes | Yes | Limited | Yes | Pay per check |
| Postmark | DMARC tool free | No | Yes (DMARC digest) | No | Yes | Pay per email |
How to run an inbox-placement test
A systematic test beats random spot-checks. Here's the workflow:
Step 1 — Verify authentication first. Before testing inbox placement, confirm your SPF, DKIM, and DMARC records are valid. A failed authentication check makes inbox-placement results meaningless — fix the foundation first. Run a free domain check or use MXToolbox to validate.
Step 2 — Choose your seed-list tool. GlockApps and Everest provide managed seed lists. You send your test email to their addresses and they report where it landed per provider. Pick a tool whose seed list covers the providers your audience actually uses.
Step 3 — Prepare test variants. Send at least two versions: one plain-text transactional message and one HTML marketing message. This isolates whether content formatting triggers spam filters independent of your domain reputation.
Step 4 — Send at realistic cadence. Don't blast all seeds simultaneously from a domain that normally sends 50 emails per day. Match your normal sending pattern to avoid triggering volume-based filters.
Step 5 — Read the report. Focus on provider-by-provider results. Gmail inbox but Outlook spam? That points to content or formatting issues (Outlook's filters weight HTML structure differently). Spam across all providers? That's likely a domain or IP reputation problem.
Interpreting seed-list results
Seed lists have limitations. A 20-seed panel won't catch every edge case. Regional providers (T-Online, Mail.ru, Yandex) need dedicated seeds. Self-hosted seed addresses on your own domains test your sending infrastructure but not provider-side filtering. Treat seed-list results as directional signals, not absolute verdicts. Combine them with Google Postmaster data and your ESP's bounce/complaint reporting for the full picture.
Diagnosing common deliverability problems
When a tool flags a problem, here's what to do:
SPF failure — your sending server's IP isn't listed in your SPF record. Check which service sent the failing email, then add its include: mechanism. Watch for the 10-lookup limit — exceeding it causes SPF to fail silently. Run dig TXT yourdomain.com and count the include and redirect entries.
DKIM failure — the signature doesn't verify. Common causes: your ESP rotated DKIM keys and the DNS record wasn't updated, or an intermediary (mailing list, forwarder) modified the message body. Check that the DKIM selector in the email header matches a published DNS record with a valid public key.
DMARC failure — means both SPF and DKIM failed (or their domains don't align with the From header). Start with p=none and DMARC aggregate reports to identify which senders are failing before tightening to quarantine or reject.
Blacklist listing — check which list flagged you using DomainCare's blacklist check or MXToolbox. Each blacklist has its own delisting process — most require you to submit a removal request after fixing the underlying issue (compromised account, open relay, spam complaint spike). Major lists like Spamhaus require direct contact; smaller lists often auto-expire after 24–48 hours.
Content filtering — if authentication passes and you're not blacklisted but still landing in spam, the problem is likely content. Shorten URLs, remove excessive images, avoid trigger phrases, and check your HTML-to-text ratio. Mail Tester's content analysis helps pinpoint specific triggers.
Connecting deliverability checks to continuous monitoring
One-off tests tell you how things look today. Continuous monitoring tells you when things break tomorrow. The gap between tests is where deliverability problems hide — an SPF record that gets accidentally deleted during a DNS migration, a new blacklist listing from a compromised form handler, a DKIM key that expires silently.
API-driven automation closes this gap. Set up scheduled DNS checks and email deliverability monitoring that run every few hours and alert on changes. DomainCare's API lets you integrate domain health checks into existing workflows — CI/CD pipelines for transactional mail services, weekly reports for client domains, or Slack/webhook alerts when authentication records change.
A practical monitoring stack for most teams:
- DomainCare — continuous SPF/DKIM/DMARC/BIMI monitoring + blacklist checks across 49 sources, with alerts via email, Slack, Discord, or webhook
- Google Postmaster Tools — Gmail-specific reputation and spam rate data
- GlockApps or Mailtrap — periodic seed-list tests before major campaigns
- Your ESP's built-in reporting — bounce rates, complaint rates, engagement metrics
This combination covers authentication health (continuous), inbox placement (periodic), reputation signals (ongoing), and delivery mechanics (per-send) without overlap or gaps.
Frequently asked questions
What's the difference between a deliverability checker and an inbox-placement test? A deliverability checker validates your domain's configuration — DNS records, authentication, blacklist status. An inbox-placement test sends actual emails to seed addresses and reports where they landed (inbox, spam, missing). Checkers tell you if your setup is correct; placement tests tell you if your emails actually arrive.
How many seed addresses do I need for a reliable test? At minimum, cover the top 4 providers your audience uses (typically Gmail, Outlook, Yahoo, Apple Mail). GlockApps uses panels of 30–70+ seeds; Everest exceeds 100 globally. More seeds give more confidence, but even a 20-seed panel across major providers catches most issues.
What should I do first if a tool reports SPF/DKIM/DMARC failures?
Fix SPF first (it's the simplest), then DKIM (coordinate with your ESP for key rotation), then DMARC (start with p=none to collect reports before enforcing). Don't jump to p=reject until you've confirmed all legitimate senders pass both SPF and DKIM.
How can I tell if the problem is content versus reputation? Send a plain-text message with no links or formatting. If it lands in inbox, the issue is content-related (HTML structure, links, trigger words). If plain text also hits spam, the problem is domain or IP reputation.
How often should I run deliverability checks? Authentication and blacklist monitoring should be continuous (every 6–12 hours). Seed-list inbox-placement tests are best run before major campaigns, after DNS changes, after switching ESPs, or on a monthly cadence for high-volume senders.
Can I use deliverability tools alongside my ESP? Yes — and you should. ESPs report on delivery mechanics (bounces, opens, complaints) but not on inbox placement or domain authentication health. Use a monitoring tool like DomainCare for the domain layer and your ESP for the sending layer.
Which integrations should I look for? At minimum: webhook or API support for alerting, DNS record validation, and reporting. If you manage multiple domains, look for tools with per-domain dashboards and bulk API operations. DomainCare's API supports all check types programmatically.