SMTP Tester is a free online tool that helps developers, system administrators, and email teams test and verify any SMTP server. Instead of guessing why mail will not send, you can watch the entire SMTP conversation happen live and see exactly where it succeeds or fails.
What the tool does
When you run a test, SMTP Tester opens a real connection to the mail server you specify and walks through the Simple Mail Transfer Protocol step by step: it reads the server greeting, sends EHLO to discover capabilities, upgrades the session with STARTTLS or implicit TLS, authenticates with the method you choose, and optionally submits a test message. Every client and server line is streamed back to your browser as a live transcript, with passwords and authentication payloads automatically redacted.
Why we built it
Most legacy SMTP testers show a single pass/fail message and leave you guessing. We wanted a tool that:
- Shows the full protocol transcript in real time, not just a result.
- Surfaces TLS diagnostics — negotiated protocol, cipher, certificate issuer, subject, and expiry.
- Supports the authentication methods real servers use — Auto, PLAIN, LOGIN, and CRAM-MD5.
- Works with any server, from Gmail, Microsoft 365, SendGrid, and Amazon SES to self-hosted Postfix, Exim, KumoMTA, and PowerMTA.
- Respects your privacy by design.
How it works under the hood
- A handshake-only mode lets you verify connection, TLS, and credentials without delivering a message — useful on production systems.
- A separate TLS probe captures certificate details that command-line tools like telnet cannot show.
- Sensible defaults select the right security mode for common ports (465 implicit TLS, 587/2525 STARTTLS, 25 plaintext).
It is a no-install, browser-based alternative to debugging SMTP with telnet, openssl s_client, or swaks, and it works even on networks where outbound port 25 is blocked.
Privacy first
SMTP Tester has no database and no user accounts. The credentials you enter are used only to connect to your server for a single test and are never stored on our servers. Saved connection profiles live only in your own browser. For full details, see our Privacy Policy.
Open to feedback
We are continually improving the tool and the library of setup and troubleshooting guides. If you have a feature request, found a bug, or want a guide for a provider we have not covered yet, we would love to hear from you — see the Contact page.