Formspree
formspree.io
Gives a static site's contact form somewhere to POST that emails you the answers
Free tier
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mxtoolbox.com
Looks up MX, SPF, DKIM and DMARC records and names the one that is wrong.
Enter a domain and it queries the mail-related DNS records, checks the syntax of your SPF and DMARC policies, tests the mail server's connectivity and banner, and checks the sending IP against about a hundred blacklists. The blacklist check is the one people arrive for, usually mid-incident. Diagnostics are explained rather than just reported, which makes it usable by people who are not mail administrators.
AltFTool does this too
If you would rather not leave the site, Email Authentication Checker covers the same job here — free, no account, and it runs in your browser.
Open Email Authentication CheckerQuestions
A genuinely usable free tier with paid plans for volume or team features. Anonymous lookups are rate-limited, and monitoring, alerting and bulk checks all need a paid account.
Yes. MXToolbox sends your input to its own servers to process it. That is fine for public material; for anything containing personal data, use one of the on-device tools instead.
Anonymous lookups are rate-limited, and monitoring, alerting and bulk checks all need a paid account.
Alternatives
Same category or same task, ordered by how closely they overlap.
formspree.io
Gives a static site's contact form somewhere to POST that emails you the answers
toolbox.googleapps.com
Paste raw email headers and see every hop, the delay at each, and the auth results
mail-tester.com
Send one email to the address it gives you and get a spam score out of ten
mailerlite.com
Sends a newsletter to a small list on a free tier that does not expire
mailinator.com
Any address at mailinator.com already has an inbox you can read without signing up
mailtrap.io
Catches the emails your app sends in a fake inbox so no real person gets them