How to use MX / SMTP Test
The SMTP Test connects to a mail server and walks through the early stages of an SMTP conversation to confirm the server is reachable, responds correctly, and is willing to accept mail for a given domain. Diagnosing email delivery problems is hard without seeing how the receiving server actually responds, and this tool surfaces exactly that: the connection result, the greeting banner, supported features and any errors. Use it to verify a newly configured mail server accepts connections, to debug why mail to a domain bounces, or to confirm a server is not an open relay.
- Enter the mail server hostname or the recipient domain.
- Click Test to open an SMTP connection on port 25.
- Read the greeting banner and the server’s responses.
- Check the supported commands and any error codes returned.
- Use the responses to pinpoint where delivery fails.
What an SMTP conversation reveals
An SMTP exchange begins with the server sending a greeting banner, followed by an EHLO handshake in which it advertises features such as STARTTLS for encryption and authentication support. The test then probes whether the server will accept mail for the target recipient. Each step returns a numeric response code — codes in the 200s mean success, while 400s signal a temporary problem and 500s a permanent rejection. Reading these codes tells you precisely where in the flow delivery breaks, turning a vague "mail bounced" into an actionable diagnosis.
| Code range | Meaning |
|---|---|
| 2xx | Success |
| 4xx | Temporary failure — retry later |
| 5xx | Permanent failure — rejected |
Common problems an SMTP test catches
A connection that times out points to a firewall blocking port 25 or a server that is down. A greeting followed by a rejection of the recipient often means the address does not exist or the domain’s mail routing is misconfigured. If the server accepts mail for arbitrary external recipients it may be an open relay — a serious misconfiguration that invites blacklisting. Seeing STARTTLS advertised confirms the server supports encrypted delivery. Each of these is far easier to spot by watching the live conversation than by guessing from a bounce message alone.
Glossary
- SMTP
- Simple Mail Transfer Protocol, used to send and relay email.
- EHLO
- The SMTP greeting command in which a server advertises its features.
- STARTTLS
- An SMTP feature that upgrades a connection to encryption.
- Open relay
- A server that accepts mail for any recipient, often abused for spam.
- Response code
- A numeric SMTP status indicating success or the type of failure.
Related reading
Frequently Asked Questions
Why use MX / SMTP Test?
- Connect to a mail server and observe its live SMTP responses in real time
- Confirm STARTTLS is advertised before trusting a server with encrypted delivery
- Detect an open relay configuration that would expose the server to blacklisting
- Diagnose the exact SMTP stage where delivery fails using numeric response codes
Common use cases
- Verify a newly configured mail server accepts connections on port 25
- Check whether a receiving server supports STARTTLS before sending sensitive email
- Diagnose a bounce caused by a recipient-rejection at the RCPT TO stage
- Test whether an on-premises mail server is reachable after a firewall rule change
- Confirm an open relay is closed after hardening a previously misconfigured server
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