Open Relay Tester
Test whether a mail server is an open relay by attempting an external-to-external send. Free, over a real SMTP connection — no email is actually delivered.
Test for an open relay
Enter a domain. We connect to its mail server (MX, port 25) and attempt an external-to-external relay.
No email is actually delivered — the transaction is abandoned after the RCPT TO response. Only test servers you are authorised to check.
What this open relay tester does
Common use cases
- 1Auditing your own mail server for misconfiguration
- 2Confirming a server refuses unauthorised relaying
- 3Checking a new SMTP setup before going live
- 4Verifying a firewall or policy change
- 5Investigating why an IP was blacklisted
- 6Security-testing infrastructure you own
Related tools
About open relays
An open relay is a mail server that accepts and forwards email from any sender to any recipient, even when neither belongs to its own domains. Spammers hunt for open relays to launder mail through someone else’s server — which quickly gets that server blacklisted.
This tool connects to the server and tries to relay from an external address to another external address. A secure server rejects it (a 5xx "relay denied" reply); an open relay accepts it. Note that port 25 is often blocked on outbound networks, so run this where you can reach the server, and only test infrastructure you are authorised to check.
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Got Questions? We Have Answers.
What is an open relay?
An open relay is a mail server that forwards email from any sender to any recipient, even when neither belongs to its own domains. Spammers abuse them, which quickly gets the server blacklisted.
How does the test work?
It connects to the domain's mail server and attempts to send from an external address to another external address. A secure server refuses this (a 5xx "relay denied" reply); an open relay accepts it.
Does the test send a real email?
No. The transaction is abandoned right after the RCPT TO response, so nothing is delivered — the tool only reads whether the server would have accepted the relay.
Why does it use port 25?
Relaying happens on port 25, the server-to-server mail port. Note that many networks block outbound port 25, so run the test where you can reach the server.
Is it legal to test a server?
Only test infrastructure you own or are authorised to assess. Probing third-party servers without permission may breach acceptable-use policies.
My server is an open relay — how do I fix it?
Configure it to relay only for authenticated users or trusted networks, and reject mail where neither sender nor recipient is local. Then re-test and check you are not blacklisted.