Open Relay Test
An open relay lets anyone send mail through your server — the fastest way to get your IP blacklisted. Enter your mail server and we'll check it. Safe test: no message is ever sent (we stop before the DATA command).
What is an open relay?
An SMTP server is an open relay when it forwards mail from any sender to any recipient — including external-to-external mail it has no reason to handle. Spammers scan for these and pump their traffic through them, so an open relay's IP and domain hit the blocklists within hours.
How this test works (and why it's safe)
We connect to your server, say EHLO, then try MAIL FROM and RCPT TO
using two external addresses your server shouldn't handle. We read the response code and stop.
The DATA command — the one that actually transfers a message — is never sent, so
nothing is relayed or delivered. If the server accepts that external-to-external recipient,
it's an open relay.
Found an open relay? Fix it
- Require SMTP AUTH (over TLS) for mail submission, and only from authenticated users.
- Refuse relaying for unauthenticated external senders.
- Accept port-25 inbound only for domains you actually host.
Already listed because of it? See how to get delisted and the deliverability checklist.
Catch a misconfiguration before spammers do
Deliveradar can re-test your mail server on a schedule and alert you on Telegram and email the moment it starts relaying or lands on a blacklist.
Start monitoring, freeFree plan, no card, no expiry.
See also: blacklist check · deliverability guides.