How to Check Email Deliverability: A Practical, Free-First Method

Last updated July 2026

Most people check deliverability backwards. They buy a tool, run a test, get a score out of ten, and still have no idea why their mail is filtered. The data that actually governs your sending is published by Google and Microsoft, for free, and almost nobody reads it. Here is the order I run through when a sending domain misbehaves.

How do I check my email deliverability?

Run five checks in order: authentication (SPF, DKIM, DMARC), your spam complaint rate in Google Postmaster Tools and Microsoft SNDS, a seed inbox placement test, your infrastructure (blacklists, reverse DNS, TLS, domain age and ramp), and finally your own send data (bounces, replies, complaints). Each step only matters if the one above it passed.

Step 1: Check authentication, because it is binary and free

Authentication either passes or it does not. There is no partial credit, and no amount of copywriting rescues a domain that fails it. Google's sender guidelines are the primary source. The short version:

• Every sender must set up SPF or DKIM, and must transmit over a TLS connection.
• Bulk senders (Google defines this as roughly 5,000 or more messages a day to Gmail addresses) must set up SPF, DKIM and DMARC, and must support one-click unsubscribe for marketing and subscribed mail.

Check it in two places. First the DNS records for your sending domain: an SPF TXT record that includes the service you send from, a DKIM public key at your platform's selector, and a DMARC record at _dmarc.yourdomain.com. Second, and this is the check people skip, send a message from your live sending setup to a Gmail mailbox you control and view the raw source. The Authentication-Results header will read something close to spf=pass ... dkim=pass ... dmarc=pass. If any of them say fail, none, or softfail, stop. Nothing below is worth measuring until that is fixed.

Alignment is the trap. SPF can pass on the envelope sender while the visible From domain is something else, which means DMARC still fails. The header tells you the truth, a DNS lookup tool only tells you a record exists. For the record-by-record walkthrough, see our guide to improving inbox placement.

Step 2: Read your actual reputation from the mailbox providers

Google Postmaster Tools and Microsoft SNDS are first-party dashboards. They are not estimates or proxies. They are the mailbox providers telling you directly what they think of your domain.

Postmaster Tools shows a spam rate: the share of your delivered Gmail messages that recipients marked as spam. Google's published guidance is to keep that number below 0.3%, and Google recommends staying under 0.10%. That is the single most important figure in email deliverability, and no third-party tool can move it for you. A vendor can score your subject line. It cannot un-press the "report spam" button in someone's Gmail.

To use Postmaster Tools you verify the domain with a DNS TXT record, then wait. It needs volume before the charts populate, so a domain sending a handful of messages a day may show nothing. SNDS works on IP addresses rather than domains, so it is most useful when you send from your own SMTP infrastructure and know your sending IPs.

Step 3: Run a seed placement test, and be honest about what it tells you

A seed test sends a copy of your message to test mailboxes across Gmail, Outlook, Yahoo, and others, then reports where each copy landed: inbox, promotions, or spam. It is the fastest way to see that a template is fine at Gmail but filtered at Outlook.

What it does not tell you is how real prospects experience your mail. Seed mailboxes are not your prospects. They never open, never reply, and never mark you as spam, so they generate none of the engagement signals that drive your reputation. Treat a seed test as a smoke alarm, not a health report. Run one on your current template with our free deliverability and spam test, and see what the paid options measure in our rundown of email deliverability tools compared.

Step 4: Check the boring infrastructure

1. Blacklists. Check the sending domain and IP against the major public blocklists. A listing on a widely used list hits delivery immediately.
2. Reverse DNS. Your sending IP should resolve to a hostname, and that hostname back to the IP. Missing rDNS is a classic silent filter at Outlook.
3. TLS. Google requires it. Confirm your SMTP connection negotiates TLS rather than falling back to plaintext.
4. Domain age and warm-up. A domain registered last week that starts sending 300 emails a day is a textbook spam pattern.
5. Volume shape. Steady growth reads as a real business. A flat line followed by a spike reads as a burner.

Step 5: Look at your own send data

Bounce rate comes first, and the distinction matters. A hard bounce means the address does not exist, and a pile of them is a list quality problem, not a reputation problem, though it becomes one fast if you keep sending. A soft bounce is temporary: a full mailbox, a server hiccup, a greylist. We cover this in our piece on the difference between a soft bounce and a hard bounce. If hard bounces are climbing, verify the list before you touch anything else.

Reply rate is an underrated deliverability metric. Replies are the strongest positive signal a mailbox provider can see. Complaint rate is the negative twin, and the one Google publishes a threshold for.

What is a good email deliverability rate?

There is no honest single number, because "deliverability rate" gets used to mean two different things. The only hard, published threshold you can anchor to is Google's: keep your Postmaster Tools spam complaint rate below 0.3%, and ideally under 0.10%. Every other benchmark you read is somebody's sample, not a law.

As a practitioner rule of thumb, and labeled as such because it is not published data: hard bounces should be low enough that you never think about them, and if seed tests start showing spam placement at a provider you were previously inboxing at, treat that as a real signal even when your delivery rate looks unchanged. That last part deserves its own section.

What is the difference between delivery rate and inbox placement?

Delivery rate measures whether the receiving server accepted your message. Inbox placement measures whether a human ever saw it. A message that is accepted and dropped straight into the spam folder still counts as delivered, which is why a very high delivery rate can sit on top of a campaign that is failing completely.

Delivered is not read. Your platform reports delivery because that is what the SMTP conversation tells it, and that conversation ends the moment the receiving server says "accepted". Whatever the filter does next is invisible to the sender. The only ways to see placement are seed tests, replies from real prospects, and the engagement data in Postmaster Tools. If delivery is high and replies are zero, assume you are in spam until proven otherwise.

How do I know if my emails are going to spam?

Three signals, in order of reliability. Your Postmaster Tools spam rate rises or your Gmail domain reputation drops. Your reply rate falls off a cliff while your delivery rate stays flat. A seed test shows spam placement at a specific provider. Any one of those is enough to act on.

The cheapest manual version: send your live template, from your live sending account, to Gmail, Outlook, and Yahoo mailboxes you control, and see where it lands. Do not send to a colleague on the same domain, because internal mail skips most of the filtering. Check the spam folder as well as the promotions tab.

Spam at one provider but not another usually means content or authentication alignment. Spam everywhere usually means reputation or infrastructure, so go back to steps 1, 2, and 4.

Is Google Postmaster Tools free?

Yes. Google Postmaster Tools is free, and so is Microsoft SNDS. Neither has a paid tier, a seat limit, or a trial. You verify ownership (a DNS TXT record for Postmaster Tools, an IP authorization step for SNDS) and you get the data. The only cost is that both need meaningful volume before they show anything useful.

This is why the tool-first instinct is expensive. Before you pay for any deliverability product, you should already know your Gmail spam rate and domain reputation, because a paid tool that tells you "your reputation is poor" has just sold you something Google gives away. Paid tools earn their keep on top of that data, not instead of it.

How do I check my sender reputation?

Sender reputation is not one score, it is a set of provider-specific judgments. Check the Gmail side in Google Postmaster Tools, which rates domain and IP reputation. Check the Microsoft side in SNDS, which reports IP status and complaint data. Then check public blocklists. That combination is as close to a real reputation picture as anyone gets.

Ignore any tool that hands you a single reputation score out of 100 across all providers. No such number exists on the receiving end. Gmail and Outlook hold separate opinions of you, and a domain can sit comfortably at one while being filtered at the other.

Reputation is earned, not configured. Authentication makes you eligible. Complaint rate, engagement, and consistent volume are what actually move you. On a new domain the reputation you are checking barely exists yet, which is what domain warm-up is for: building send history before the volume arrives.

Put it in a loop

Deliverability is a monitoring job, not a one-time audit. Check Postmaster Tools weekly, run a seed test whenever you change your template, domain, or list source, and re-check authentication any time you add a sending account. The failures that hurt are rarely sudden. They creep, and they show up in your spam rate before your reply rate.

One last thing, because it gets lost in deliverability work: landing in the inbox is only half the job. If your campaign points people at a website, that page has to carry the conversation, and it is worth auditing that page's copy, layout, and call to action before you scale up sending volume. There is no point winning the filter fight and then losing the click.