Free Open Relay Checker
Full conversation with the mail server
Enter a domain or the address of a mail server. We connect to its mail port and check if it would pass on mail from a stranger to an outside address. No email is sent during the test.
Free Open Relay Checker Overview
What is an open relay?
A mail server normally has two jobs. It accepts mail for its own domains, and it sends mail for its own users after they log in. An open relay also accepts mail from anyone on the internet and forwards it to any other domain, without asking who is sending. Years ago many servers worked like this. Today it is treated as a serious security hole.
Why does this test matter?
Spammers scan the internet all the time looking for open relays. When they find one, they push thousands of messages through it, and every message carries your server's IP address instead of theirs. Soon that address appears on block lists such as Spamhaus. Gmail, Outlook and other providers then reject or hide your normal mail, so invoices, replies and password resets stop reaching people. The extra load can also slow the server down, and some hosting companies suspend accounts that send spam. A quick test shows the problem before spammers find it.
How does this checker test a server?
We look up the mail server (MX record) of the domain you enter, connect to it on port 25 and hold a short conversation, the same way another mail server would:
- We wait for the server to greet us.
- We say hello with EHLO and read the list of features it offers.
- We give an outside sender, relaytest@dnsinfo.live.
- We ask it to take a message for an outside recipient, relaytest@gsldns.net. Neither address belongs to the domain you are testing.
- We cancel with RSET and leave with QUIT.
We never send the DATA command, so no message is delivered at any point. The report shows every line of the conversation together with a plain explanation of each step.
What do the results mean?
- Green, not an open relay: the server refused the outside recipient, usually with a code such as 550 or 554 and a message like "Relay access denied". This is the answer you want.
- Red, open relay: the server accepted the outside recipient with a 250 code. Close it as soon as possible.
- Orange, inconclusive: the server refused our test sender, asked us to try later with a 4xx code (often greylisting) or did not finish the conversation in time. The report says exactly where it stopped. Run the test again in a few minutes.
The report also points out other weak spots visible from outside: missing reverse DNS, no encryption (STARTTLS), password login offered before encryption, the VRFY and EXPN commands that reveal valid addresses, and a greeting that shows the mail software version.
When should I run this test?
Run it after setting up a new mail server, after changing relay or firewall settings, after moving to a new host, and whenever your IP address lands on a block list for no clear reason.
Open Relay Troubleshooting and Practical Fixes
Postfix forwards mail for strangers when its relay rules are too loose or mynetworks covers too many addresses.
Exim relays for any address listed in its relay host lists, so one wrong entry is enough to open it.
A receive connector that faces the internet gives anonymous users permission to send to any recipient.
Admins often open relaying for a whole network range so that one device can send, and that range is too wide.
The server postponed the test or refused our sender, so the check could not finish. Greylisting servers do this on purpose for unknown senders.
FAQs
smtpd_relay_restrictions to permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination and keep mynetworks to addresses you trust. In Exim, keep relay_from_hosts to localhost and your own servers. In Microsoft Exchange, remove the ms-Exch-SMTP-Accept-Any-Recipient permission for anonymous users on connectors that face the internet.