Test the message, not just the records
There are two very different things people mean by "testing deliverability." One is reading your published DNS to confirm SPF, DKIM, and DMARC exist and are well-formed. The other is sending a real message and seeing how a receiving server actually judged it. The first is necessary and quick. The second is the one that catches the problems that hurt, because alignment, reputation, and content only show up once a message is in flight.
A useful test sends from the system you use for production mail, then reads the verdict the receiver wrote. That is the difference between knowing your records look right and knowing your mail gets in.
Step 1: get authentication right first
A delivery test is only meaningful once the records are in place. Publish SPF and DKIM for the sending domain, add a DMARC record, and make sure the domain in your visible From header aligns with the SPF or DKIM domain. If any of that is missing or misaligned, fix it before you test anything else, because it will dominate the result. Email authentication explained covers how the three records work together, and the instant snapshot on the home page checks all of them for a domain in one pass.
Step 2: read the Authentication-Results header
When a receiver accepts your message it records what it checked in an Authentication-Results header, defined in RFC 8601. Open the raw message at the receiving end and look for it. You want three results: spf=pass, dkim=pass, and dmarc=pass.
The detail that trips people is alignment. A message can show dkim=pass while the header.d value belongs to a different domain than your From address, which means the signature is valid but does not represent you, so DMARC still fails. Check that the DKIM signing domain and the SPF domain line up with the domain a recipient sees. Always read this header from the receiver's copy of the message, since the receiver is the one that writes it; your outbound copy does not have it.
Step 3: check blacklists and content
Authentication can pass while delivery still fails, and the two usual culprits are reputation and content. Look up the sending domain and IP against public DNS blacklists with a blacklist lookup; a listing on a widely used list is enough to push mail to spam on its own. Then look at the message itself for the patterns filters dislike: image-only bodies with little text, bulky or broken HTML, links to domains with poor reputation, and bulk mail without a working unsubscribe.
Step 4: read inbox placement
Authentication tells you a message is trusted to be from you. It does not tell you where the message landed. Inbox placement testing fills that gap by sending to seed addresses across Gmail, Outlook, Yahoo, and others and reporting which delivered to the inbox, which went to spam, and which vanished. It matters most for bulk and marketing senders, where reputation and engagement decide placement after authentication passes. A small transactional sender chasing one bounce can usually skip it and rely on the header and blacklist checks.
Step 5: watch reputation over time
A single test is a snapshot. Reputation is a trend, so connect the continuous tools too. Google Postmaster Tools shows your domain and IP reputation, spam-complaint rate, and authentication results for mail to Gmail. Microsoft's Smart Network Data Services does the same for the sending IP into Outlook.com and Microsoft 365. Watching these means you see a reputation slide while you can still act on it, rather than discovering it when a campaign underperforms.
The fastest way to run the whole test
Doing all of this by hand for every send is slow. The email deliverability test runs the message-level checks in one place: you send a real email to a unique address, and it reads the Authentication-Results for SPF, DKIM, DMARC, and alignment, checks blacklist status, and surfaces content issues, so you get the receiver's-eye view without assembling it yourself. Pair it with the home-page snapshot for the DNS side, and you have covered both halves: the records that should be right and the message that proves they are.
If your mail is already going to spam and you want the causes in order rather than just a test method, start with why are my emails going to spam. For the full set of instruments and how to read each one, see email deliverability tools.