SMTP tester

Most “email not sending” bugs die at a specific SMTP step — the TLS upgrade, the credential, the rejected recipient — but mail clients hide which one. This tool runs the conversation in the open: every command, every server reply, every step's timing. Point it at your own mail server or at the Tempx capture gateway (prefilled) and read the transcript like a packet log.

SMTP host

Port 587 — STARTTLS 2525 — STARTTLS (alt)

Username (optional)

Password (optional)

Mail from

Rcpt to

Run SMTP test

Credentials are sent over HTTPS, used once for this dialogue in memory, never stored or logged, and redacted in the transcript. Ports tested: 587 and 2525 (STARTTLS).

Reading the transcript

• Greeting / EHLO — is anything listening, and what does it advertise? Missing STARTTLS or AUTH lines here explain most later failures.
• STARTTLS — the connection upgrades to TLS before credentials are sent; a failure here means a broken or mismatched certificate.
• AUTH — shown only when you provide credentials; 535 means wrong username or password.
• RCPT TO — a 550/551/553 here is the server refusing the recipient (relay denied, unknown mailbox, policy) even though your settings are fine.
• DATA — a 250 after the message content is the finish line: the server accepted the mail for delivery.

Testing the Tempx gateway? The captured message lands in the credential's paired inbox a second later — the whole loop, end to end. Setup details: SMTP integration, concept: SMTP testing.