Deliverability tooling is a confusing category because the products sound alike and measure very different things. Teams commonly end up with three tools reporting overlapping information and no answer to the question that matters, which is where their mail is landing right now.
Six categories. What each one actually tells you, and the order to buy them in.
1. Authentication checkers
What they measure: whether SPF, DKIM, DMARC and MX are present and correctly formed on a domain.
What they do not: whether mail is reaching the inbox. Perfect authentication and terrible placement coexist easily.
When you need it: at setup, and whenever you add a domain. Also worth running periodically, because records get edited and break silently.
What to look for: SPF lookup count against the ten-lookup limit, DKIM key length, DMARC policy and whether reporting addresses are set, and a check that the tracking domain resolves with valid SSL.
Free tools cover this adequately. There is little reason to pay, and a domain health checker that grades the whole setup at once is more useful than four separate lookups.
2. Inbox placement tests (seed tests)
What they measure: where mail actually lands — Primary, Promotions, Updates or Spam — across a panel of test addresses at different providers.
What they do not: how your real recipients behave. A seed panel is a sample, not your list, and seed addresses do not engage the way people do.
When you need it: before launching a campaign, after any infrastructure change, and on a schedule thereafter. This is the single most valuable deliverability measurement and the one most teams skip.
What to look for: per-provider results rather than a single score, coverage of the providers your prospects actually use, and enough tests included that you can run them regularly rather than rationing them.
A note on Promotions: landing there is not the same as spam and for some audiences it is survivable. For cold B2B outreach it usually is not, because the tab does not get checked with the same attention.
3. Blocklist monitors
What they measure: whether your domain or IP appears on published blocklists.
What they do not: the internal reputation systems at Gmail and Microsoft, which matter considerably more than public blocklists and are not published anywhere.
When you need it: ongoing, as an alert rather than a dashboard. Being listed is worth knowing within hours.
What to look for: which lists are checked, alerting rather than passive display, and guidance on delisting. Bear in mind that on shared infrastructure delisting may not be yours to do.
4. Reputation monitors
What they measure: provider-reported signals, primarily Google Postmaster Tools data — domain reputation, IP reputation, spam rate, authentication success.
What they do not: anything about Microsoft, which has no comparable public equivalent, or about placement in specific folders.
When you need it: once you have enough Gmail volume for the data to populate, which takes a few hundred messages a day to the same domain.
Google Postmaster Tools is free and underused. If you send meaningful Gmail volume and have not set it up, do that before buying anything.
5. Content and spam-word checkers
What they measure: whether your copy contains patterns associated with spam, and sometimes a composite score.
What they do not: how filters will actually treat your message, because modern filtering weights sender reputation and engagement far more heavily than content. Content matters least of everything on this list.
When you need it: occasionally, as a sanity check. Useful for catching genuinely bad habits — excessive links, all-caps subject lines, attachments in a first touch.
A caution: do not let a content score send you rewriting copy when the real problem is authentication or reputation. This is the most over-weighted tool in the category precisely because it is the easiest to act on.
6. Warmup
What it does: builds sending history on new mailboxes through network activity.
Covered at length in the standalone warmup piece. Short version: most platforms now include it, often uncapped, and a separate tool is usually duplicate spend.
The overlap map
| Tool | Setup | Ongoing | Answers “why is mail in spam?” |
|---|---|---|---|
| Authentication checker | Essential | Periodic | Partly — rules out one cause |
| Inbox placement test | Essential | Scheduled | Yes, tells you where and on which provider |
| Blocklist monitor | Optional | Alerting | Sometimes |
| Reputation monitor | Once at volume | Ongoing | Partly, Gmail only |
| Content checker | Optional | Rare | Rarely |
| Warmup | Essential | Continuous | No, it prevents rather than diagnoses |
Buying order if you have nothing
- Authentication checking. Free, takes ten minutes, rules out the most common cause outright.
- Warmup. Check whether your sending platform already includes it before buying anything.
- Inbox placement testing. The measurement everything else depends on. Without it you are guessing about the one thing that matters.
- Google Postmaster Tools. Free, and the closest thing to a provider telling you what it thinks of you.
- Blocklist alerting. Useful as a notification, rarely worth a subscription on its own.
- Content checking. Last, and occasional.
Most teams buy this list roughly backwards, starting with content because copy feels like the controllable variable.
The diagnostic sequence
When placement drops, work in this order rather than changing several things at once:
Has anything changed in DNS? Records get edited. Check first. What does a placement test say, by provider? If it is one provider and not others, that narrows it enormously. What is the bounce rate? Above 3% and the list is the cause, not the infrastructure. What is the complaint rate? Above 0.1% and targeting or copy is the cause. Did volume jump recently? A sudden increase looks like a compromised account. Are you on a blocklist? Check, but expect it to be a symptom rather than the cause.
Changing copy comes after all of that, not before.
The short version
Authentication checkers tell you your setup is valid. Placement tests tell you where mail lands. Reputation monitors tell you what Google thinks. Blocklist monitors tell you if you are listed. Content checkers tell you the least of the five.
If you own one thing, make it placement testing, because everything else is a hypothesis until you have measured where mail is actually going.
And work the diagnostic in order. Most spam-folder problems resolve at DNS or list quality, long before anyone needs to rewrite a subject line.
Next: the deliverability guide, or SPF, DKIM and DMARC explained.
Frequently asked questions
What is the best tool to check email deliverability?
An inbox placement test, because it measures where mail actually lands by provider. Authentication checkers confirm your records are valid, which is necessary but does not tell you about placement.
How often should I run an inbox placement test?
Before launching a campaign, after any infrastructure change, and on a regular schedule thereafter — weekly for high-volume senders, monthly at lower volumes.
Do spam word checkers actually work?
Marginally. Modern filtering weights sender reputation and recipient engagement far above content. They catch genuinely bad habits but will not fix placement problems caused by authentication or list quality.
What is Google Postmaster Tools?
A free Google service reporting domain reputation, IP reputation, spam rate and authentication success for mail sent to Gmail. It needs a few hundred messages a day to populate and is the closest thing to direct feedback from a provider.