SMTP Relay Check
Detect whether an SMTP server allows open relay.
Configuration
Common ports: 25, 587, 465
Test Options
Security Status
Test Summary
Security Checks
Connection Info
Learn more & FAQ
What does an open relay check do?
An open relay lets anyone, anywhere, send mail through your server to any destination - a misconfiguration spammers specifically scan for. This tool connects to your mail server on its SMTP port and attempts a series of relay tests that a real open relay would accept, without actually sending any mail, to tell you whether it's exposed.
How to read your results
A properly configured server rejects every relay attempt that isn't from an authenticated or explicitly allowed source, and this tool will show that as a pass. If any test succeeds, your server is currently relaying mail for arbitrary senders - treat that as urgent, since an open relay tends to get discovered and abused for spam quickly once it's live.
Frequently asked questions
What is an open mail relay?
An open relay is a mail server that accepts and forwards email from anyone, for any destination, without requiring authentication or restricting it to known senders. It's a misconfiguration, not a feature - a properly configured mail server only relays mail for its own authenticated users or verified domains.
Why is an open relay dangerous?
Spammers actively scan for open relays specifically because they let anyone send mail through someone else's server for free, using that server's reputation. Once discovered, an open relay is quickly abused for mass spam, which gets the server's IP blacklisted and can take down legitimate mail delivery for its real owner.
How do I fix an open relay once it's found?
Configure your mail server (Postfix, Exim, Exchange, etc.) to require authentication for outbound relay, and restrict which IP ranges or domains it will relay for. Every major mail server software documents this as a standard, well-supported configuration step.
Can a mail server be partially open - relaying for some sources but not others?
Yes, and that's actually the correct, secure configuration - relaying for your own authenticated users and known internal IP ranges while rejecting everyone else. The problem is specifically when a server relays for arbitrary, unauthenticated, external senders.
Does fixing an open relay affect legitimate email I send?
No - closing an open relay only removes the ability for unauthenticated outside senders to use your server. Mail from your own authenticated users and systems continues to work exactly as before.