SMTP Test
Connect to a mail server, check banner, STARTTLS & open relay.
Enter a value above to run SMTP Test
e.g. mail.example.com
Authenticated / end-to-end test
Log in to a mail server and, optionally, probe a recipient. Credentials are sent over POST and never stored, cached, or put in the shareable link.
Quick answer
SMTP Test connects to a mail server, reads its greeting banner, checks whether STARTTLS encryption is offered, and probes for an open relay. A healthy result shows the banner and STARTTLS support with the relay test closed; an open relay is a serious misconfiguration that lets anyone send mail through your server.
The SMTP Test tool opens a connection to a mail server on its listening port (usually 25, 465, or 587), reads the greeting banner, negotiates the EHLO handshake, and reports which capabilities the server advertises. It checks whether STARTTLS is offered for encrypting the session and probes whether the server accepts mail for domains it does not host, which would indicate an open relay. It is meant for verifying your own outbound and inbound mail server before it goes into production, or when messages are being rejected or silently dropped.
How it works
The tool performs a TCP connection to the target host and port, then waits for the 220 service-ready banner. It sends EHLO and parses the multi-line reply listing supported extensions such as STARTTLS, AUTH, SIZE, and 8BITMIME. If STARTTLS is advertised, it can upgrade the plaintext channel to TLS. To detect an open relay, it issues MAIL FROM and RCPT TO with an external recipient and records whether the server returns a 250 acceptance or a 550 rejection.
How to read the results
A healthy server returns a 220 banner naming its hostname, then a clean EHLO list that includes STARTTLS and, on submission ports, AUTH. Codes in the 2xx range mean success, 4xx means a temporary failure worth retrying, and 5xx is a permanent rejection. If the open-relay probe is accepted with 250, the server will forward mail for anyone and must be locked down immediately. A 550 or 554 on that probe is the correct, secure response.
Common problems & fixes
A connection timeout usually means a firewall is blocking the port or the server is not listening; confirm the correct port for the role. No STARTTLS in the EHLO list means the session stays in cleartext, so enable TLS in your MTA configuration. A banner that leaks internal hostnames or software versions can aid attackers. If your provider blocks port 25 outbound, use 587 with authentication for submission instead of relaying directly.
Frequently asked questions
- Which port should I test, 25, 465, or 587?
- Use port 25 for server-to-server relay and inbound delivery testing. Port 587 is the modern submission port for authenticated clients sending mail, using STARTTLS. Port 465 provides implicit TLS from the first byte, also for submission. Test the port that matches the role you are diagnosing, since a server may behave very differently on each.
- What is an open relay and why is it dangerous?
- An open relay accepts and forwards mail from any sender to any recipient without authentication. Spammers exploit these to send bulk mail while hiding their origin, which quickly lands the relay on blocklists like Spamhaus. Once listed, your legitimate mail also gets rejected. A correctly configured server only relays for authenticated users or its own hosted domains.
- Does the test send a real email or deliver a message?
- No. The check ends the SMTP conversation during the envelope stage, before any DATA command that would transfer message content, so no actual email is delivered. The relay probe stops as soon as the server accepts or rejects the recipient. This lets you safely evaluate configuration and security without generating traffic or test messages in real mailboxes.