Help Centre

Last updated: July 29, 2026

Getting started

What is MailProbe?

MailProbe connects to a mail server over real SMTP and reports what the server says when something connects to it — the greeting banner, whether STARTTLS is offered, which authentication mechanisms are advertised, and how long the handshake took. It is the check an admin would do by hand with telnet or openssl s_client, for people who do not have a terminal.

How to run your first probe

Go to the App page.
Type the hostname of a mail server you want to test (e.g. mx.example.com or smtp.gmail.com).
Choose the port: 25 (plain SMTP), 587 (Submission, often with STARTTLS), or 465 (SMTPS, implicit TLS).
Click Check. MailProbe opens a connection, reads the banner, sends EHLO, and parses the capability list.
Review the results: the raw banner text, whether STARTTLS is offered, the advertised AUTH mechanisms, and the connection latency.

Interpreting the results

Greeting banner — The first line the server sends on connect. Often includes the software name and version (e.g. 220 mx.example.com ESMTP Postfix (Ubuntu)).
STARTTLS — If "Offered" (green), the server can upgrade the plain-text connection to encrypted via STARTTLS. If "Not offered" (amber), the server either does not support encryption or requires it implicitly on a different port.
Auth mechanisms — The SASL mechanisms the server advertises after STARTTLS (or on the plain connection). Common mechanisms include PLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, XOAUTH2. MailProbe never sends an AUTH command — it only reads the list.
Latency — The time from connect to receiving the EHLO response. High latency may indicate a slow or overloaded server.

Frequently asked questions

The app says "The probe server is not connected" — how do I fix this?

The App page requires a running backend server to perform real SMTP connections (browsers cannot open raw TCP sockets). The backend server is a separate process that must be deployed and running. If you are self-hosting, ensure the probe server is started and that DROPLET_URL is set in the frontend. If you are visiting the public site, the backend may not be running yet — this is a known limitation of the current build (see the Known Limitations section below).

What does "STARTTLS: Not offered" mean? Is my server insecure?

It means the server did not advertise the STARTTLS capability on the port you probed. On port 25, many servers do not offer STARTTLS until after EHLO — if you do not see it, try probing port 587 (Submission) which typically requires STARTTLS. On port 465 (SMTPS), the connection is encrypted from the start (implicit TLS), so STARTTLS is not needed. The absence of STARTTLS on port 25 alone does not mean your server is insecure — it may require TLS on port 587 instead.

Can I probe my Gmail, Outlook, or other public mail servers?

Yes, MailProbe connects to any hostname you type on the standard SMTP ports. However, some providers may block or throttle connections from unfamiliar IP addresses. Always ensure you have permission to probe the server — MailProbe performs a real network handshake, which may appear in the server operator's logs.

Does MailProbe send email or log in?

No. MailProbe reads public SMTP banners only. It sends EHLO (the SMTP greeting command, which every server expects) and parses the response. It never sends AUTH, MAIL FROM, RCPT TO, or DATA. No credentials are transmitted or stored. The tool does not send email.

How many probes can I run? Is there a rate limit?

The Free plan has no rate limits or usage caps — probe as many servers as you need. However, each probe opens a real outbound TCP connection from the probe server, so excessive automated use that degrades the service for other users may result in throttling. As stated in the Terms of Service, fair use applies.

My probe returned an error — what went wrong?

Common causes: (1) the hostname does not resolve to a valid mail server — verify the DNS record; (2) the port is blocked by a firewall — some networks block outbound port 25; (3) the server rejected the connection because your IP is not authorised — carrier-grade NAT or residential IPs are sometimes blocked; (4) the server timed out — slow servers may not respond within the 15-second probe timeout. Check the error message in the red error card for details.

Can I probe a server on a non-standard port like 2525?

No. MailProbe only connects to ports 25, 465, and 587. This is by design — the tool is a diagnostic probe for standard mail infrastructure, not a general-purpose port tester. Probing arbitrary ports would turn it into a port scanner, which is outside the tool's scope and prohibited by the Terms of Service.

Known limitations

MailProbe is early-stage software. The following are honest limitations of the current build:

Backend server not yet deployed. The App page and the backend probe server are built and tested, but the production backend server is not running on the public site. The frontend is functional and ready; the missing piece is the deployment of the Node.js probe server. When you see "The probe server is not connected," this is why.

Pro features are in development. The Pro tier (API access, batch probes, extended timeouts, custom EHLO, CSV/JSON exports, dedicated endpoint) is listed on the Pricing page but is not yet available for purchase. The subscribe flow is a demo that collects no payment and creates no subscription.

No monitoring, alerts, or scheduled re-checks. MailProbe runs when you click Check, and only then. There is no sender infrastructure, no notification system, and no background scheduler. We do not plan to add these — see plan.md for the rationale.

No email sending. MailProbe is a read-only diagnostic tool. It cannot send, relay, or deliver email. If your problem requires sending a test message to verify delivery, MailProbe cannot help with that.

Does not test actual authentication or encryption enforcement. MailProbe reads what the server advertises, not what it actually enforces. A server may advertise STARTTLS but still accept plain-text submission, or list PLAIN auth but require TLS for it. The tool reports capabilities, not policy.

Probe results are ephemeral server-side. The Free tier stores nothing on the server. Probe history is kept in your browser's localStorage and cleared when you clear it. If you refresh or change browsers, the history is gone.

How to contact us

MailProbe is a small project. Here is how to reach us, depending on the kind of issue:

Bug reports & feature requests

Open an issue on the project's issue tracker. Include the hostname you were probing, the port, the full error message shown in the red error card, and your browser/OS version. For feature requests, describe what you are trying to do — we may suggest a workaround.

Security concerns

If you believe MailProbe has a security issue (e.g., it is sending data it should not, or exposing internal information), contact the maintainer directly. Do not file a public issue for active security vulnerabilities.

General support questions

Check the FAQ above first. If your question is not answered here or in the Terms or Privacy pages, reach out through the project's discussion forum or support channel. We aim to respond within 1–2 business days.

Commercial / partnership inquiries

For questions about deploying MailProbe internally, white-labeling, or custom development, contact the owner directly. These inquiries are typically routed to the project maintainer rather than front-line support.

The fastest way to get help is to provide as much context as possible in your first message: what you probed, what you expected, what you got, and which browser you are using.