Help Centre

Getting started, answers to common questions, and what to expect from smtpprobe today.

Getting Started

What is smtpprobe?

Smtpprobe opens a real SMTP connection to any mail server and reads what the server says. It shows you the server's greeting banner, whether it offers STARTTLS for encryption, which authentication mechanisms it advertises, and how quickly it responded.

How to use it

  1. Go to the Probe page. No account or sign-up needed.
  2. Enter a hostname. Type the hostname of the mail server you want to test — for example mx.example.com, mail.yourcompany.com, or aspmx.l.google.com. Just the hostname; no http:// prefix, no path.
  3. Choose a port. Select the port you want to probe:
    • Port 25 (plain) — The standard SMTP port. Most mail servers listen here for incoming mail from other servers.
    • Port 587 (submission) — The mail submission port. Clients and users typically send outgoing mail through this port.
    • Port 465 (TLS) — SMTP over TLS. The connection is encrypted from the start, before any SMTP commands are exchanged.
  4. Click "Probe". The tool will open a connection, read the banner and capabilities, and display the results.
  5. Read the results. Each finding is presented as a card: connection status, SMTP banner, STARTTLS availability, authentication mechanisms, handshake timing, and a full list of advertised capabilities. You can also expand the raw SMTP transcript to see the exact conversation.

Interpreting your results

Banner. The first line the server sends. It usually identifies the software (Postfix, Exchange, Sendmail, etc.) and the hostname the server believes it has.

STARTTLS. If "Offered" (green badge), the server will upgrade a plain-text connection to encrypted after the STARTTLS command. If "Not advertised" (amber badge), the server doesn't offer this upgrade — any data sent after a plain-text connection would be unencrypted.

Authentication mechanisms. These are the methods the server advertises for clients to log in. PLAIN and LOGIN send credentials in the clear unless TLS is active. CRAM-MD5 and XOAUTH2 are more secure. Smtpprobe reads the list — it never attempts to authenticate.

Handshake timing. The round-trip time from SYN to the first banner line. High values (>500ms) can suggest network latency, a distant server, or a server under load.

Frequently Asked Questions

Do I need an account to use smtpprobe?

No. Smtpprobe has no accounts, no sign-up, no login. The tool answers when you ask it. Your probe results exist only in your browser during the current page session — refreshing or navigating away clears them.

Does smtpprobe send email or authenticate to my server?

No. The probe opens a TCP connection, reads the server's greeting banner, sends an EHLO command (the standard SMTP handshake that any mail client sends), and reads the server's list of advertised capabilities. It never attempts AUTH, never sends a MAIL FROM or RCPT TO command, and never transmits email data or credentials.

My probe returned "Connection refused" or timed out. What does that mean?

A "Connection refused" error means the server actively rejected the connection on that port. Possible reasons:

  • The hostname is incorrect or the server is not running on that port.
  • A firewall is blocking outbound connections to port 25, 465, or 587. Many residential and cloud-hosted networks block port 25 by default.
  • The server is configured to reject connections from unknown IPs.

A timeout means the server did not respond within the probe's timeout window. This can happen with heavily firewalled servers, servers on very slow networks, or servers that rate-limit connections from unknown IPs.

Can I test a server on a non-standard port, like port 2525 or 26?

No. For safety, smtpprobe only allows the three standard mail ports: 25 (plain SMTP), 465 (TLS), and 587 (submission). This restriction prevents the tool from being used for port scanning or other unintended purposes. If your mail server uses a non-standard port, you will need to test it with a terminal-based tool such as openssl s_client or nc (netcat).

Why is the tool showing "Demo mode" / simulated results?

Smtpprobe is in development. The probe backend that opens real TCP connections to mail servers is built but not yet deployed to production. The web app currently uses simulated results to demonstrate the interface and result display. When the real backend is connected, the demo banner will be removed and all probes will connect to actual servers. See Known Limitations below for details.

Does smtpprobe store the hostnames I probe?

No. Smtpprobe does not have a database, does not log probe requests, and does not store hostnames or results server-side. The hostname you enter is used only to establish the SMTP connection during that single probe. Your browser's IP address is visible to the probe backend during the HTTP request, as with any web service, but it is not logged or stored.

Known Limitations

Smtpprobe is a development-stage tool. These are honest descriptions of what is not yet built or still limited:

Simulated probe results

The probe backend that makes real TCP connections to mail servers is built (using only Node.js standard library — net and tls modules) but is not yet connected to the web app. All results you see today are generated client-side with mock data. The "Demo mode" banner on every page is intentional and accurate.

No scheduled or recurring probes

The tool answers when you ask it. There is no scheduler, no email alert system, and no notification mechanism. Smtpprobe cannot check your server every five minutes and email you when something changes. That is a deliberate choice — there is no email sender in this product.

Limited to three standard ports

Only ports 25, 465, and 587 are allowed. Non-standard mail ports (e.g. 2525, 26, 25000) are not supported. This is a safety constraint that also prevents misuse for network reconnaissance.

No STARTTLS upgrade performed

The probe detects whether STARTTLS is advertised but does not perform the TLS handshake itself. This means the probe does not validate certificates or test whether the TLS handshake succeeds — it only reports whether the server says it supports STARTTLS.

No IPv6 support yet

All probe connections are made over IPv4. IPv6 hostname resolution is not attempted. This will be added when the real backend is deployed.

No export, no API, no saved history

Results cannot be exported to PDF or saved between sessions. There is no API for programmatic access. These features are planned for the Pro tier but are not available yet. Results exist only in your current browser tab.

No legal entity registered

The business owner has not yet registered a legal entity or provided a registered address. The Terms of Service and Privacy Policy reflect this and will be updated when legal details are available.

Contact Us

Smtpprobe is an early-stage tool. The quickest way to get help is through the project's GitHub repository, where you can open an issue for bug reports or feature requests. For direct inquiries, the owner can be reached through the contact details provided in the repository profile.

This product does not have a support email address, live chat, or phone line at this time. Please open an issue for anything that isn't working or could be improved.