Email deliverability test: what it measures, and what it cannot

A deliverability test answers one question: when you send to a real mailbox, does the message land in the inbox, the spam folder, or nowhere at all? Everything else people call a “deliverability test” — a DNS checker, a blacklist lookup, a spam-score grader — measures something adjacent and useful, but not that.

The distinction matters because the adjacent checks are the ones that are free and instant, and they will happily tell you everything is fine while your mail sits in spam.

The three things people mean by “deliverability test”

1. An authentication check. Does your domain publish valid SPF, DKIM and DMARC records? This is a DNS lookup. It takes seconds, costs nothing, and is a prerequisite rather than a result — passing it does not mean you reach the inbox, but failing it means you often won’t.

2. A content or spam-score check. A tool scores your message against filter heuristics — typically some SpamAssassin derivative — and returns a number. Useful for catching an obvious mistake like a bare link-shortened URL or a missing text part. It is a weak proxy for the real thing, because the major providers stopped relying on static content rules a long time ago.

3. An inbox placement test. You send your actual campaign to a set of real mailboxes you control across Gmail, Outlook, Yahoo and others, then record where each message landed. This is the only one that measures placement, and it is the one that costs money.

How an inbox placement test works

The mechanism is simple. A provider maintains a seed list — mailboxes spread across the major consumer and business providers. You add those addresses to a campaign and send as normal. The provider then inspects each mailbox and reports the placement: inbox, promotions or updates tab, spam, or missing entirely.

Run the same campaign from the same domain a week later and you have a trend line, which is more informative than any single reading.

What a seed test cannot tell you

This is where most published deliverability numbers quietly mislead, so it is worth being precise.

• Seed mailboxes have no relationship with you. Gmail’s filtering is heavily personalised — it weighs whether this recipient has opened, replied to or archived your mail before. A seed mailbox that has never interacted with anyone is not a typical recipient, so its placement is a proxy, not a measurement of what your real prospects see.
• Seed lists get burned. If a seed address is widely used, providers can learn to treat mail to it differently from ordinary mail.
• Tab placement is not spam placement. Landing in Gmail’s Promotions tab is a different outcome from landing in spam, and tools vary in how they report it. Check which one a number refers to before you act on it.
• It says nothing about why. A placement test tells you that you are in spam. It does not tell you whether the cause is your authentication, your domain’s history, your sending volume, your content or your list quality.

None of this makes seed testing worthless — it is the best directional signal available, and a drop from 90% to 40% placement over two weeks is real information regardless of the caveats. But a single “98% inbox placement” figure, especially one published by a vendor about itself, is worth much less than it appears.

The free data most senders ignore

Two sources report on your actual mail to actual recipients, at no cost, and both are underused.

Google Postmaster Tools shows domain and IP reputation, spam complaint rate, authentication pass rates and delivery errors for mail you send to Gmail. It requires verifying your sending domain and a reasonable volume before it displays data, and it covers Gmail only — but for most B2B lists Gmail and Google Workspace are the majority of recipients.

DMARC aggregate reports arrive as XML at whatever address you put in the rua= tag of your DMARC record. They tell you which servers are sending mail claiming to be your domain and whether it passed alignment — which catches both misconfiguration and outright spoofing. They are tedious to read raw; several free parsers exist.

Check these before you test

A placement test on a misconfigured domain just tells you the domain is misconfigured, which a DNS lookup would have told you for free.

• SPF — a TXT record listing the servers permitted to send for your domain. Watch the ten-DNS-lookup limit; exceeding it causes a permanent error, and it is easy to exceed by chaining include: statements.
• DKIM — a cryptographic signature on each message, with the public key published in DNS. Confirm the selector your sending tool actually uses is the one published.
• DMARC — a policy record telling receivers what to do when SPF and DKIM fail alignment with the visible From domain. Start at p=none with reporting on, read the reports, then tighten.
• Blacklists — check the domain and the sending IP against the major lists. Spamhaus in particular will stop a campaign outright.
• Domain age and warmup — a domain registered last week that starts sending hundreds of cold emails is the single most reliable way to land in spam. Warmup tools exist to build engagement history before real sending starts.
• List quality — a high hard-bounce rate is read as a signal that you do not know who you are mailing. Verification before sending is cheaper than the reputation damage after.

The thresholds that are actually published

Since February 2024, Google and Yahoo have published requirements for bulk senders — those sending roughly 5,000 or more messages a day to their users. The headline requirements are SPF and DKIM and a DMARC record, one-click unsubscribe on marketing mail, and a user-reported spam rate kept below 0.3%, with 0.1% named as the level to stay under.

Most cold email operations sit below the volume threshold, but the spam-rate number is the useful part: it is a published figure, it is what the providers actually measure, and it is far stricter than most senders assume. Three complaints in a thousand is the ceiling.

If the test comes back bad

Work in this order, because the cheap causes are also the common ones: fix authentication first, then check blacklists, then reduce volume per mailbox and spread sending across more inboxes, then clean the list, then look at content. Changing subject lines before checking DKIM is the most common wasted week in cold email.

One caution on remedies: sending speed and inbox rotation are handled by your sending tool, and tools differ in how aggressively they throttle. That difference is measurable, and it is what we are building the cold email benchmark to measure — on our own domains, with the same campaign, so the numbers are comparable. We have not published those results yet, and this guide deliberately contains none: everything above is mechanism and published vendor requirements, not our findings.