How to Test Your Cold Email Deliverability
To test cold email deliverability, combine four methods: a seed test that sends real mail to inboxes across providers to see where it lands, an authentication check of your SPF, DKIM, and DMARC records, a blocklist lookup for your domain and sending IPs, and postmaster tools that report your reputation directly from the receiver. Each measures a different thing, and only the seed test reveals actual inbox placement.
Why a delivery rate tells you almost nothing
Most sending tools report a delivery rate — the share of messages the receiving server accepted instead of bouncing. A 99% delivery rate sounds reassuring but means little: accepted mail can still land in the spam folder, where no one sees it.
The number you actually care about is inbox placement: of the mail that was accepted, how much reached the inbox rather than spam or the promotions tab. Sending platforms cannot see this, because the receiver does not report it back. That gap is why deliverability has to be tested deliberately rather than read off a dashboard. For the broader picture of how placement works, see the cold email deliverability guide.
Seed tests: the only direct read on placement
A seed test (or inbox-placement test) sends your message to a set of seed mailboxes you or a tool controls, spread across Gmail, Outlook, Yahoo, and other providers. You then check each seed inbox and record where the message landed. The result is a placement breakdown by provider.
A seed test is the closest thing to a direct measurement of inbox versus spam. Run it on your real campaign copy and your live sending setup, not a stripped-down sample, because subject line, links, and volume all move placement.
What a seed test does not tell you:
- Seed inboxes have no prior relationship with you. Real recipients who have opened or replied to you may see better placement; cold recipients on a worn domain may see worse.
- It is a snapshot. Reputation shifts, so today’s pass can become next month’s spam folder.
- A handful of seeds is noisy. Use a wider spread and look at the pattern, not a single inbox.
Treat seed-test results as a directional signal across many mailboxes, never as a placement guarantee. Anyone promising a fixed inbox percentage is overstating what any test can prove.
Authentication checks: SPF, DKIM, DMARC
Before chasing placement, confirm the receiver can verify your mail. Authentication is the entry ticket — fail it and nothing else matters.
- SPF lists which servers may send for your domain. See SPF records for cold email.
- DKIM adds a signature so receivers can detect tampering or forgery. See DKIM for cold email.
- DMARC tells receivers what to do when SPF or DKIM fail, and where to send reports. See DMARC for cold email.
You can verify these by sending a message to a checking address that parses your headers, or by inspecting the headers of a received message directly (look for spf=pass, dkim=pass, dmarc=pass). Since February 2024, Google and Yahoo require bulk senders to authenticate with all three, support one-click unsubscribe, and keep their spam complaint rate low — commonly cited as under roughly 0.3%. A full walkthrough lives in the SPF, DKIM, DMARC setup checklist.
An authentication check confirms your records are valid and aligned. It says nothing about reputation or placement — a perfectly authenticated domain can still land in spam.
Blocklist checks
A blocklist (or blacklist) check looks up your sending domain and IP addresses against well-known lists that receivers consult. A listing can quietly tank delivery for the affected provider.
The check is fast and worth doing whenever placement drops without an obvious cause. It tells you whether you are listed and where; it does not explain why the listing happened, and removal often requires fixing the underlying behaviour first. For what to do about a hit, see email blacklists and cold email.
Postmaster and reputation tools
The largest receivers expose their own view of your sending. Google Postmaster Tools, for example, reports domain and IP reputation, spam complaint rate, and authentication results for mail you send to Gmail — straight from the receiver, with no inference required.
This is the most authoritative read you can get, but it has limits: it only covers that one provider, it needs enough volume to populate, and a low spam-complaint rate there does not generalise to Outlook or smaller mailbox hosts. Use it alongside seed tests, not instead of them.
What each method actually measures
| Method | What it measures | What it cannot tell you |
|---|---|---|
| Seed test | Inbox vs spam placement, by provider | Real-recipient behaviour; stays true over time |
| Authentication check | Whether SPF/DKIM/DMARC pass and align | Reputation or placement |
| Blocklist lookup | Whether your domain/IP is listed | Why it happened, or how to get delisted |
| Postmaster tools | Reputation and complaints, per receiver | Anything outside that one provider |
No single method is enough. Authentication and blocklist checks catch hard failures; postmaster tools track reputation over time; the seed test is the only one that reads actual placement. Run them together.
How often to re-test
A deliverability test is a snapshot, not a setting. Re-test before a new domain carries real volume, after any DNS, warmup, or copy change, and on a recurring cadence once live. Domains wear out under sustained sending — often within a few months — so a domain that tested clean can drift into spam as its reputation erodes. See why cold email domains burn out. A regular re-test catches a slide early instead of after a dead campaign.
How Mailionaire approaches this
Mailionaire runs each sending domain on its own isolated Microsoft 365 tenant with SPF, DKIM, and DMARC configured automatically, so the authentication side of a test passes by default. Monitoring watches sending health and replaces mailboxes and domains as they wear out, which keeps the underlying infrastructure honest between your own placement tests. Pricing is a flat $50 per active domain per month, with no per-mailbox metering. See how it works for the full setup.
FAQ
What is the best way to test cold email deliverability?
Run a seed test: send your real campaign to a spread of seed inboxes across Gmail, Outlook, and other providers, then record where each one landed. It is the only method that measures actual inbox-versus-spam placement. Pair it with an authentication check and a blocklist lookup for the full picture.
Does a seed test prove I will land in the inbox?
No. A seed test shows placement for those specific seed mailboxes at that moment. They have no engagement history with you, so real recipient inboxes can differ. Treat the result as a directional signal across many seeds, not a guarantee, and re-test periodically.
How often should I test deliverability?
Test before a new domain carries volume, after any DNS or warmup change, and on a recurring cadence once live — weekly or before each major send is reasonable. Domains wear out under sustained volume, often within a few months, so a single passing test does not stay true.
Does passing a deliverability test mean my cold email is legal?
No. Deliverability testing is purely technical and says nothing about whether you are permitted to send. Consent and local law remain your responsibility. Germany's UWG §7(2) generally requires prior opt-in for advertising email, including B2B. This is not legal advice.
Mailionaire provisions real, isolated Microsoft 365 mailboxes for cold email — built in Switzerland, with optional EU/Swiss data residency — then monitors and replaces them as they wear out. One flat price per domain. See how it works →