All tools
Each tool answers one question about a domain and runs in your browser. Start with the DNS lookup for any record type, or use the email checks when mail is rejected: SPF, DMARC and DKIM. If you only have an error message, paste it into the bounce code lookup. The guides explain how to fix what a check reports.
Lookups
DNS LookupLook up A, AAAA, CNAME, MX, NS, TXT, SOA, CAA and more over DNS-over-HTTPS, with TTL and DNSSEC flag. Free, no logs.MX LookupSee the MX records of any domain by priority, their IP addresses and common misconfigurations (CNAME targets, unresolvable hosts, null MX).Reverse DNS (PTR) LookupFind the hostname for an IPv4 or IPv6 address and verify forward-confirmed reverse DNS (FCrDNS), the check mail servers rely on.
Email authentication
SPF Record CheckerValidate an SPF record against RFC 7208: counts the 10 DNS lookups, walks every include, flags +all, multiple records and syntax errors.DMARC Record CheckerParse and validate the DMARC record at _dmarc.yourdomain per RFC 9989: policy strength, np, t, rua/ruf, alignment modes and external report authorization.Email Header AnalyzerPaste raw email headers to see the delivery path hop by hop, where delays happen, and whether SPF, DKIM and DMARC passed and aligned. Runs offline in your browser.SMTP Bounce Code LookupPaste a bounce message or enter an SMTP code such as 550 5.7.26 to see what it means at Gmail, Microsoft 365 or Yahoo, why it happens and how to fix it. Runs in your browser.Email Provider Setup CheckCheck whether your domain's MX, SPF include, DKIM selectors and DMARC match what Google Workspace, Microsoft 365, Zoho, Fastmail and others document. Runs in your browser.DKIM Record CheckerLook up the DKIM key at selector._domainkey for any domain: follows CNAMEs, decodes the RSA key size, checks t=, h= and revoked keys against RFC 6376 and 8301.