About
Our server opens a brief SMTP conversation with the host and port you give it, reads the greeting, notes whether STARTTLS is advertised, and hangs up. Nothing is delivered to anyone, which makes this safe to run against a production mail server.
How to use
Enter the host, for example smtp.gmail.com, and the port. If you only know the domain, run the MX lookup first to find the host that actually accepts its mail.
What the greeting tells you
An SMTP server announces itself on connection with a line naming its software and hostname. That line is a fast sanity check: it confirms something is listening and it often reveals that the host answering is not the one you expected, which happens whenever a domain's mail was moved to a provider and old configuration was left behind. The banner is public and reading it changes nothing.
STARTTLS, and what advertising it means
A server offering STARTTLS is telling clients it can upgrade the connection to TLS. Seeing it here means the capability is advertised on that port. It does not tell you whether the certificate is valid, whether the cipher choice is sensible, or whether a client will actually complete the upgrade. Treat it as one line in a longer configuration story.
Reading a timeout
A refused connection is usually honest: nothing is listening. A timeout is more often filtering, and the two most common sources are a firewall dropping the packet and a provider blocking the port outbound. When you are debugging mail that will not send from a virtual machine, testing port 25 from here and from the machine itself is how you tell whose block it is.
Where this fits when mail is not being delivered
Start with the MX lookup to learn where mail for the domain should go. Test that host here to confirm it answers. Only then look at authentication, SPF and DKIM alignment, and the sending logs, because each of those assumes the connection works. Following that order saves debugging an authentication problem that was really a blocked port.
FAQ
Does it send a test message?
No. It connects, reads the greeting, and disconnects. Nothing is queued and no recipient is contacted.
Which port should I test?
587 for submission with STARTTLS, 465 for implicit TLS, and 25 for server to server delivery. Cloud providers block outbound 25 from customer machines almost universally, which is the single most common cause of mail that will not leave a new server.
Can it check my username and password?
No. This stops before authentication. It answers whether the server is listening and what it offers, which is a separate question from whether your credentials work.
The connection times out from here but works on my laptop.
Then the server is filtering by source address, or your network path differs. Both are normal, and a test from one vantage point cannot describe another.
Related tools
- Email tools
- Gmail Address Checker
Check up to ten Gmail addresses per run.
- Outlook Mail Reader
Read recent mail using your own refresh token.
- MX Lookup
Mail servers and priorities for a domain.
- Email Header Analyzer
Parse Received hops and auth lines locally.
- Open Port Checker
Try one TCP connection to a host and port.