Help Centre

Q1. What hostnames can I check?

Any publicly reachable mail server hostname. Enter the domain or MX hostname you would configure in an email client or mail transfer agent — for example mail.example.com, mx.google.com, or smtp.office365.com. smtppeek connects from its own server, so it must be able to reach the target host over the internet.

Private/internal hosts (hosts on your local network, behind a VPN, or using RFC 1918 addresses like 192.168.x.x) cannot be checked — the tool has no access to your internal network. Use a terminal-based client inside your network for that.

Q2. What do the different results mean?

Banner. The first line the server sends when you connect. It usually shows the server software (e.g. Postfix, Exim, Microsoft Exchange), the version, and the hostname.

Capabilities. SMTP extensions the server supports. Common ones include PIPELINING (send multiple commands at once), SIZE (max message size), 8BITMIME (8-bit content), DSN (delivery status notifications), and ENHANCEDSTATUSCODES (richer error codes).

STARTTLS. If the server lists STARTTLS in its capabilities, it supports upgrading a plain connection to encrypted. If it does not, the server either does not support TLS or requires implicit TLS (port 465).

Auth methods. Authentication mechanisms the server advertises (e.g. PLAIN, LOGIN). These only indicate what the server says it supports — they are not a security assessment. smtppeek does not attempt to authenticate.

Timing. The total time from socket open to receiving the full capability list. High values may indicate network latency, rate-limiting, or a slow server.

Q3. Why can't I check my internal mail server?

smtppeek connects from its own backend server, which is on the public internet. It has no route to private or internal networks. To check an internal mail server, use a command-line tool like openssl s_client -connect or telnet from a machine inside the same network.

This is a deliberate design constraint — smtppeek will never probe private address space or accept arbitrary port ranges.

Q4. Does smtppeek store my data?

No. smtppeek does not store, log, or retain any data from your checks. Each request is ephemeral — the hostname, port, and results exist only in your browser session. Once you close or refresh the page, the results are gone. There are no user accounts, no sign-up, no cookies for tracking, and no stored history.

See the privacy notice for full details.

Q5. Why did my check fail?

Connection failures generally fall into a few categories:

Timeout (10 seconds). The server did not respond. The host may be firewalled, the port may be blocked, or the server may be down. Try a different port — many servers listen on 587 or 465 but not 25.

Connection refused. The server actively rejected the connection on that port. Verify the port number is correct and the mail service is running.

Host not found. The hostname could not be resolved via DNS. Check for typos (e.g. gmail.com instead of smtp.gmail.com).

Connection reset. The server closed the connection during the SMTP handshake. This can happen if the server detects unusual behaviour or has strict connection policies.

If you are sure the server is reachable and speaking SMTP on the port you specified, the issue may be a transient network problem — try again in a few seconds.

Q6. Does smtppeek check my email or send messages?

No. smtppeek is a read-only diagnostic tool. It opens a TCP connection, reads the greeting banner, sends an EHLO command (the SMTP equivalent of "hello, what do you support?"), and closes the connection. It does not send email, deliver messages, authenticate, or modify anything on the target server.

Q7. Is there an API I can use programmatically?

The app page communicates with a backend API (GET /api/peek?host=...&port=...) that returns JSON. This endpoint is intended for the app's own use and is rate-limited. There is not yet a documented public API with keys, authentication, or SLAs — these are planned for the Pro tier.

If you automate requests to the app's API, please respect the rate limit. Excessive automated use may be blocked without notice.

Q8. What is the rate limit?

The Free tier has a rate limit to ensure fair access for everyone. The exact threshold is not publicly documented and may be adjusted. If you encounter a rate-limit response, wait a few seconds before trying again. A Pro tier with no rate limit is planned.