Most free tools in the deliverability space are free as in "works until you need it". Rate-limited after three tests, degraded during peak hours, timing out when Gmail is slow. We think that pattern is lazy. Here is the SLA we commit to on the free tier, how we measure it, and what you actually get by paying — which is a shorter list than you might expect.
Free tier: 99.9% uptime, under 2 minutes median result time, 10 tests per hour per IP, 20+ seed mailboxes, 30 days of history, no signup, no credit card. Paid tier pays for priority processing, webhooks, longer retention, and HD screenshots — not for the core verdict.
The SLA numbers
Commitments we publish and measure every month:
- Uptime: 99.9% measured across the public API and the web UI. Roughly 43 minutes of downtime per month, budgeted.
- Result time: median under 2 minutes, 95th percentile under 5 minutes. Measured from
start_testtostatus=complete. - Rate limits: 10 tests per hour per IP, 50 per day. Generous enough to test a campaign; tight enough to keep the seed network healthy.
- Provider coverage: 20+ seed mailboxes — Gmail, Outlook, Yahoo, Mail.ru, Yandex, GMX, T-Online, ProtonMail, iCloud, Fastmail, and more rotating in. At least 3 Gmail, 3 Outlook and 3 Yahoo seeds available at any time.
- Data retention: 30 days on the free tier. After that, test records are deleted; seed mailbox bodies are deleted at the same cadence.
- Support: community via email and the docs search. No guaranteed response time. We usually reply within a day.
How we measure uptime
Two independent probes, one hosted on a separate cloud and region from the primary infrastructure. Both run every minute. A minute counts as down if either probe reports failure and both of the following are true for that minute:
- The public web UI returns non-2xx or takes longer than 10 seconds to render the landing page.
- A synthetic placement test started against a canary sender either fails to start or does not reach
status=completewithin 10 minutes.
We publish monthly numbers at /status. Numbers are raw — no rounding, no generous interpretation of "partial availability".
What happens when we breach
If we breach 99.9% in a calendar month, we publish a post-mortem within 5 business days describing the root cause, the minutes affected, and the changes we are making. The post-mortem goes on the status page and is linked from the changelog.
We do not give credits on the free tier. A credit on a free tool is theatre. What we give is transparency — the same post-mortem you would get from a paid SaaS at 100× the price. If you are running something critical on top of the free tier and need financial protection, the paid tier exists and comes with service credits against the real SLA.
Why free SPF/DKIM/DMARC/DNSBL is not a gimmick
The most common question we get: "what's the catch?". Free deliverability tools are usually loss leaders for upsell into a paid product — the free test is a trailer for the paid feature. Our model is different. The paid tier does not unlock a better placement test. The placement test is the same code path, the same seed mailboxes, the same scoring engine. Free and paid run side by side on the same infrastructure.
So what is the catch? We monetise through three paths that do not degrade the free tier: a paid API for automated monitoring, a paid MCP server tier with higher quotas, and custom-branded reports for agencies. None of those three require taking anything away from the anonymous user who just wants to check a single campaign.
Where the paid tier adds real value
Honest list. This is not a gated-feature pitch:
- Priority processing. Free-tier tests sit behind a fair-share queue. Paid tests skip it. At peak, the gap is 30–60 seconds.
- Higher rate limits. 1,000 tests per hour on the standard paid plan, up from 10 on free. Needed for CI and monitoring workloads.
- Webhooks. Push notifications on test completion, auth changes, and blacklist events. Free tier is polling-only.
- 90-day retention. Up from 30 days on free. Essential if you care about month-over-month placement trends.
- HD screenshots. Free tier gets compressed thumbnails; paid tier gets full-resolution PNGs suitable for client reports.
- Email support with 24-hour target response. Free tier is best-effort; paid tier is a named inbox with a target SLA.
- Team accounts, audit logs, SSO. For anyone who needs to share the tool internally without sharing an API key.
Better placement verdicts. A different scoring engine. Hidden providers. A higher-quality test. The verdict you see on the free tier is identical to the one paid customers see. That is the point: the data is the data.
Comparison vs GlockApps
GlockApps caps the free tier at three tests per month, after which every feature — authentication lookup, DMARC analysis, even viewing historical results — requires a paid plan. That is a fine business model, but it is not ours. The comparison in concrete numbers:
- Free tests per month: Inbox Check ~240 (10/hour for 24 hours is the theoretical cap; in practice you use what you need). GlockApps 3.
- Free auth check: Inbox Check unlimited. GlockApps gated.
- Signup required to see your own DNS: Inbox Check no. GlockApps yes.
- Free DNSBL coverage: Inbox Check 50+ lists. GlockApps limited.
- Retention on free: Inbox Check 30 days. GlockApps 7 days.
This is not a criticism of GlockApps — their paid tier is genuinely excellent. It is to explain what the trade-off looks like and why someone in a pinch should bookmark both.