WORKS WITH ALL EMAIL PROVIDERS
This Mail Tester was built by Henry Timmes, a named contributor to RFC 7489, the IETF standard that defines DMARC, now required by Gmail, Yahoo, and Microsoft for bulk senders. That foundational work directly informed how this tool evaluates your email.
Most mail testers simply run your email through SpamAssassin and give you a score. This tool goes further, checking your SPF alignment, DKIM signing, DMARC policy, blacklist status, and compliance signals the way a mailbox provider actually evaluates incoming mail. Because understanding how those systems work from the inside is exactly what a decade of email authentication work provides.
Henry also founded ZeroBounce, one of the world's leading email validation platforms, along with Unlock The Inbox, Postbox US, and Campaign Cleaner, each built around the same principle: that deliverability is never one thing. Your content, your data, your infrastructure, and your reputation are all either working for you or against you, depending on how you manage them.
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What each check actually means, and why it matters for inbox placement.
SPF is a DNS record that tells the world which IP addresses are allowed to send email for your domain. When your message arrives at Gmail, Yahoo, or Microsoft 365, the first thing they do is check whether the sending IP is on that list. If it's not there, that's a red flag. The most common reason SPF fails is that senders add a new ESP, like SendGrid or HubSpot, and forget to add it to their SPF record. The other one is exceeding 10 DNS lookups, which breaks SPF entirely even if everything looks correct. Use ~all while you're troubleshooting and switch to -all once you're confident your record is complete.
DKIM puts a cryptographic signature in every email you send. The receiving mail server pulls your public key from DNS and uses it to confirm two things: that the email actually came from your domain, and that nobody modified it in transit. If DKIM is missing or the signature doesn't validate, that's a trust problem. Gmail, Yahoo, and Microsoft all require DKIM for bulk senders now. Most ESPs will handle the signing automatically once you add a CNAME record to your DNS. If your test is showing a DKIM failure, start there.
DMARC is what ties SPF and DKIM together. It also tells receiving servers what to do when something fails: none means monitor only, quarantine means send it to spam, and reject means block it outright. The part people get tripped up on is alignment. DMARC doesn't just need SPF or DKIM to pass; it needs the domain in your From header to match the domain that passed those checks. You can pass both and still fail DMARC if the domains don't line up. This happens a lot when ESPs use their own Return-Path domain. The fix is getting your ESP to sign with DKIM using your domain. DMARC also sends you reporting data on who is sending mail that claims to be from you, which is useful for spotting spoofing.
The spam score comes from SpamAssassin, which looks at your subject line, HTML, links, headers, and text-to-image ratio and assigns points for anything that looks suspicious. Under 1.0 is where you want to be. Above 4.0 and you're in trouble. Above 5.0 and SpamAssassin considers it spam. The common culprits are all-caps subject lines, excessive exclamation points, broken or missing unsubscribe links, and sketchy URLs. One thing to understand though: spam score only tells you what SpamAssassin thinks. Gmail, Yahoo, and Outlook each run their own filters on top of that, and those systems weight sender reputation and engagement heavily. A clean score alone won't save you if your domain has a reputation problem.
Blacklists are real-time databases that track IP addresses and domains with a history of sending spam. Mail servers query these on every inbound message. If your sending IP or domain shows up on Spamhaus, Barracuda, SORBS, or SpamCop, your email can fail across multiple providers at once with no other explanation. The usual causes are high complaint rates, spam trap hits from old or purchased lists, or a mail server that got compromised. Getting delisted means going to each blacklist operator's website individually and submitting a request. Spamhaus will not delist you until the underlying problem is actually fixed, not just claimed.
Authentication and spam score are not the whole picture. The HTML in your email is evaluated too. Missing unsubscribe links will get you flagged by Gmail and Yahoo, both of which now require them for bulk senders. Heavy image-to-text ratios, broken links, deeply nested table structures, and URLs pointing to low-reputation domains all add up. Our Campaign Cleaner platform checks for all of this automatically and fixes the issues in one click, so you are not hunting through your HTML manually before every send.
Questions we hear from senders who can not figure out why their email is going to spam.
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