Help Centre

MailProbe supports the three standard SMTP ports defined in the RFCs:

• 25 (SMTP) — The original SMTP port used for server-to-server mail relay. Many residential ISPs and cloud providers block outbound port 25 to prevent spam.
• 465 (SMTPS) — SMTP over TLS from the moment the TCP connection opens. Used by some legacy systems and configured receivers.
• 587 (Submission) — The modern mail submission port. Most email clients (Outlook, Thunderbird, Apple Mail) use this to send outgoing mail.

We restrict to these three standard ports because MailProbe is a mail-server diagnostic tool. If you need to test a non-standard port, you would need to use a different tool (e.g. openssl s_client or nc from a terminal).

STARTTLS is the mechanism by which a plain-text SMTP connection upgrades to an encrypted (TLS) connection. If the server does not advertise STARTTLS in its EHLO response, it means:

• The server does not support encrypted connections on that port.
• Email sent over that connection will travel in plain text, readable by any network in between.
• For port 465 (SMTPS), TLS is established before any SMTP commands are exchanged, so STARTTLS is not advertised — the encryption is already in place.

If STARTTLS is missing on port 25 or 587, consider this a security concern. Modern mail servers should offer STARTTLS alongside a valid certificate.

No. MailProbe performs a read-only handshake. It:

• Opens a TCP connection (plain or TLS depending on the port).
• Reads the server's 220 greeting banner.
• Sends EHLO mailprobe — a standard SMTP command that asks the server "what do you support?"
• Receives the multi-line EHLO response.
• Closes the connection with QUIT.

MailProbe never sends a MAIL FROM, RCPT TO, or AUTH command. No email is transmitted and no credentials are exchanged. The tool is intentionally limited to banner reading and capability discovery.

Probe failures usually fall into one of these categories:

• DNS resolution failure — The hostname you entered does not resolve to any IP address. Double-check the spelling. Some providers use different hostnames for incoming vs. outgoing mail servers (e.g. imap.example.com for receiving, smtp.example.com for sending).
• Connection refused or timeout — The server is not listening on the port you selected, or a firewall is blocking the connection. Try a different port (especially 587 instead of 25). Many residential ISPs and cloud platforms block outbound port 25.
• No SMTP greeting — The server accepted the TCP connection but did not send a valid SMTP banner within the timeout. This can happen if the server speaks a different protocol on that port (e.g. a web server listening on port 25).
• EHLO rejected — The server responded with an error to the EHLO command. Some servers require HELO (the older command) instead, or have strict access controls.

Check the raw exchange log in the probe results — it shows exactly which bytes were sent and received, which can help pinpoint the issue.

You should only probe mail servers you own, operate, or have explicit permission to test. Probing someone else's server without authorisation may violate computer fraud and abuse laws in many jurisdictions (e.g. the CFAA in the United States, the Computer Misuse Act in the UK).

MailProbe is a diagnostic tool intended for:

• Verifying a mail server you have just configured.
• Diagnosing delivery problems with your own mail infrastructure.
• Checking that changes to your server's configuration are reflected in its advertised capabilities.

MailProbe records the hostname and IP address of every target it probes for operational logging and abuse prevention. Probing unauthorised targets may result in your access being blocked.

No. Probe results are ephemeral. They are generated by the backend, returned to your browser, and displayed on the page. They are not stored in any database, logged to disk, or sent to any third party. When you reload the page or navigate away, the results are gone.

Your IP address is handled as described in our Privacy Policy — it appears in standard HTTP server logs retained for up to 14 days for rate-limiting and abuse detection, and then rotated automatically.