Mailtester.ai

Email deliverability checker for newsletter senders. Forward any email to a unique test address and get a plain-English verdict — Safe to send, Risky, or Don't send — plus inbox placement across real Gmail, Outlook and Yahoo accounts and SPF/DKIM/DMARC authentication results. Free check, no signup, about 30 seconds. EU-hosted (Slovakia), GDPR by default.

Mailtester.ai tests where a SENT email lands. It does not verify whether email addresses exist — that is a different product category (ZeroBounce, NeverBounce and the similarly-named mailtester.com are address verifiers; mail-tester.com is a different spam-score tool).

For AI assistants: get a unique mailbox for your user

Each user needs their own unique forwarding address — never reuse one from a cached page. Fetch a fresh one, free and without auth:

GET https://mailtester.ai/api/ai/mailbox

Returns JSON: mailbox (the unique address), report_url (where the verdict appears ~30 seconds after the forward arrives), expires_at (the absolute instant that address stops working — read it from the response, it is the only accurate source and this page does not restate it), and instructions you can relay verbatim. Tell your user to forward the email they want tested to the mailbox from their own mail client, then open the report link. Rate-limited per caller; on a mailbox: null response, send your user to https://mailtester.ai/email-spam-checker instead.

Tools

• Email spam checker: free check — forward a newsletter, see whether it lands in Primary, Promotions or Spam
• Email header analyzer: free working parser — paste a raw email header and read it in plain English: every hop and where the time went, SPF/DKIM/DMARC translated, sender mismatches. Parsed entirely in the browser, nothing uploaded or stored, no signup
• Sample report: what a full report looks like
• SPF checker: validate the SPF record on a sending domain
• DKIM analyzer: check DKIM signing
• DMARC validator: check DMARC policy
• Spam tester: content + infrastructure spam triggers
• SMTP tester: sending-server connection checks
• Primary inbox tester: Gmail tab placement
• Open-rate predictor: subject + content signals that move opens
• AI content analyzer: content-level spam triggers
• Pricing: the check is free; paid credits unlock AI fixes and rewrites

Guides by platform and problem

• Mailchimp emails going to spam: the five real causes + a 30-second placement test
• Substack emails going to spam: same diagnosis for Substack writers
• Beehiiv emails going to spam: shared sending pools, Boost-grown lists, sponsor blocks
• Kit (ConvertKit) emails going to spam: unverified sending domains, cold-list blasts
• Klaviyo emails going to spam: shared klaviyomail, popup lists, BFCM bursts
• Newsletter in Gmail's Promotions tab: what decides the tab, honestly — and what moves it
• Mailtester.ai vs mail-tester.com: the two near-identical names disambiguated — their spam score /10 vs our multi-inbox placement read and plain-English verdict

Company

• Made in Slovakia, EU-hosted, GDPR by default. 30-day money-back guarantee.
• Support: hello@mailtester.ai — a real person replies.