SMTP Diagnostics
Test a mail server's SMTP ports, banner, STARTTLS support and reverse DNS.
Probes SMTP ports 25, 465, and 587 — reads the banner, sends EHLO, and detects STARTTLS support.
About this tool
This tool connects to a mail server on the three standard SMTP ports — 25 (server-to-server), 465 (implicit TLS submission) and 587 (STARTTLS submission) — and reports whether each accepts connections, the SMTP banner it greets with, whether STARTTLS is offered, and the server's reverse DNS.
Use it to debug "connection refused" errors from mail clients, verify a new mail server before pointing MX records at it, or confirm TLS is available so credentials and mail are never sent in plaintext.
Frequently asked questions
What is the difference between ports 25, 465 and 587?
Port 25 is for server-to-server delivery and is often blocked by ISPs. Port 587 is the standard submission port for mail clients, upgrading to TLS via STARTTLS. Port 465 is submission over implicit TLS (encrypted from the first byte). Clients should use 587 or 465.
What is STARTTLS?
STARTTLS upgrades a plaintext SMTP connection to an encrypted one. The client connects normally, the server advertises STARTTLS in its EHLO response, and the session switches to TLS before authentication. Servers without it expose credentials in plaintext.
Why does the SMTP banner matter?
The banner (the 220 greeting line) should contain the server's real hostname, matching its reverse DNS. Mismatched or generic banners are scored negatively by some receiving servers and complicate troubleshooting.