SMTPDoctorfree forever · no signup · runs in seconds
Fix email deliverability
before it breaks trust.
Validate SPF, DKIM, and DMARC, trace DNS propagation, and catch security exposures — the diagnostics MXToolbox charges for, free.
Email Authentication
DNS Records
Diagnostics
Security
TLS & Certificates
Free email & DNS diagnostics, without the paywall
SMTPDoctor is a free toolkit for diagnosing the things that quietly break email deliverability and domain security. Every check runs live against real DNS and mail infrastructure — no cached guesses, no signup, no limits. Whether an important message landed in spam, a DNS change isn't taking effect, or you're hardening a domain before launch, these tools show you exactly what's configured and what to fix.
Start with email authentication
The three records that decide whether your mail is trusted are SPF, DKIM, and DMARC. Use the SPF Checker to validate your sending policy and catch the 10-lookup limit that silently breaks SPF, the DKIM Checker to confirm your signing keys are published, and the DMARC Checker to see how strictly spoofed mail is handled. For a single overview, the Domain Reputation Checker scores all of it together with concrete fixes.
Diagnose DNS, TLS, and website security
Beyond email, SMTPDoctor covers the full picture: trace how a change has spread with the DNS Propagation Checker, inspect a site's certificate with the SSL / TLS Certificate Checker, audit response headers with the Security Headers Checker, and test for exposures like open zone transfers and open mail relays. Every tool includes a plain-English explanation of what it checks and how to fix what it finds.