An SMTP test opens a TCP connection to a mail server and walks through the Simple Mail Transfer Protocol conversation the same way a real mail client would. It reads the server greeting, sends EHLO to discover capabilities, optionally secures the session with STARTTLS or implicit TLS, authenticates if credentials are supplied, and can submit a real message with MAIL FROM, RCPT TO, and DATA.
Each step proves something specific. A successful connect and greeting prove the host and port are reachable. EHLO reveals the advertised extensions (AUTH mechanisms, SIZE, STARTTLS). A completed STARTTLS or implicit TLS handshake proves encryption works and exposes the certificate. A 235 after AUTH proves your credentials are valid. A 250 after DATA proves the server accepted the message for processing.
Important: SMTP acceptance (a 250, sometimes with a queue id) is submission success, not proof of inbox delivery. After acceptance, delivery can still fail or land in spam because of DNS/MX issues, greylisting, or missing SPF, DKIM, and DMARC alignment. To check end-to-end, send to a mailbox you control and review your server logs for that queue id.
SMTP Tester runs this whole sequence from the browser and streams every client and server line into a live, credential-redacted transcript—so you can see exactly which step fails instead of reading a generic "could not send" error. It is a no-install alternative to debugging with telnet, openssl s_client, or swaks.
What a test checks
- Connectivity: host and port reachable
- Encryption: STARTTLS or implicit TLS handshake and certificate
- Authentication: AUTH PLAIN / LOGIN / CRAM-MD5 success (235)
- Submission: server accepts the message (250, often with a queue id)
- Handshake-only mode skips sending for safe production checks