SMTP Test
Connect on port 25 and test banner, reverse DNS, STARTTLS and open relay.
Enter a target
e.g. example.com veya mail.example.com — press ⏎ / ⌘⏎
What is SMTP Test?
SMTP Test opens a real SMTP session on port 25 and inspects the server's protocol-level behaviour: greeting banner, greeting time, ESMTP extensions advertised via EHLO, STARTTLS support and the certificate behind it, forward-confirmed reverse DNS, and open-relay status.
The open-relay check issues MAIL FROM and RCPT TO, reads the reply and then RSETs — DATA is never sent, so no message is transmitted. An open relay gets found within minutes and lands the server on major blocklists within hours.
Failing to connect to any host means one of two things: the servers really are unreachable, or the network running this tool blocks outbound port 25. Most cloud providers block it by default because of spam, so the result should not be read as "server down".
Frequently Asked Questions
11 questionsDoes this test send email?
No. The open-relay check issues MAIL FROM and RCPT TO, reads the reply and then RSETs the session. DATA is never sent, so no message is ever transmitted. This is the standard, non-abusive way the test is done.
Why is an open relay so critical?
An open relay lets anyone send mail to the world through your server. It gets found within minutes, and the result is usually your server hitting the major blocklists within hours and all your legitimate mail being blocked. Close it immediately.
What does "couldn't connect to any host" mean?
Two things this tool can't tell apart: the servers really are unreachable, or the network running this tool blocks outbound port 25. Most cloud providers block it by default because of spam. Don't read it as "servers down" — verify from another network.
Is a banner/PTR mismatch a problem?
It isn't a hard rule — even large providers differ (Gmail's banner says mx.google.com while PTR is 1e100.net). Some spam filters give a small bonus for matching, which is why it's shown as a warning rather than an error.
What if STARTTLS isn't supported?
All mail to that server travels unencrypted and can be read in transit. Senders using MTA-STS or DANE may refuse delivery entirely. STARTTLS should always be enabled on a modern mail server.
Why does greeting time matter?
Some servers delay the greeting deliberately to deter spam (greylisting/tarpitting). Delays over 5 seconds can time out legitimate senders too; above 10 seconds you risk real delivery loss.
How often should I run this against my own server?
After every configuration change and after certificate renewals. For ongoing coverage use Monitors, which alerts you when certificate expiry or blocklist status changes.
My server uses port 587 — why test 25?
587 is the submission port and requires authentication; server-to-server delivery always uses 25. This tool connects to 25 because that's what the world sees when delivering mail to you.
How many MX hosts are tested?
The first three by preference. Each is reported separately, because backups are often configured differently from the primary — and the real problems usually turn up on the backup nobody looks at.
What do the EHLO extensions mean?
The ESMTP features the server supports: STARTTLS for encryption, SIZE for maximum message size, PIPELINING for command batching, 8BITMIME for 8-bit bodies, AUTH for authentication. STARTTLS and SIZE are the ones that matter for delivery.
Could the open-relay test get my server blocklisted?
No. It sends one MAIL FROM and one RCPT TO and then resets; no message is transmitted and your server hasn't sent spam. What gets a server listed is an open relay actually being abused.