SMTP Open Relay Test

This is an SMTP open relay test script which is able to optionally report open relays to DSBL (Distributed Server Boycott List). Should you discover any bugs or problems, kindly let us know. Please do NOT abuse the system.

Hostname or IP address: 
  report open relays to DSBL (Distributed Sender Blackhole List)

SMTP Open Relay Test – Frequently Asked Questions

What is an open relay?

An open relay (sometimes also referred to as a third-party relay) is a mail server that does not verify that it is authorised to send mail from the email address that a user is trying to send from. Therefore, users would be able to send emails originating from any third-party email address that they want.

Why is an open relay bad?

Spammers are constantly on the lookout for open relay SMTP (Simple Mail Transfer Protocol) servers. They use the open relays to send unsolicited mail to a large number of email addresses, which has an impact on delivery speed, bandwidth, disk storage space, CPU processing and more.

A number of organisations are cracking down on spam originating from open relay servers by forming blacklists. Several anti-spam solutions check if the mail server is in one of these blacklists before allowing incoming mail from it. If your mail server gets listed on one of these blacklists, a lot of the emails that you would be sending would not be reaching their destination.

What software blocks spam from open relays?

If your open relay is found by a blacklist organisation (usually if someone reports spam coming from your mail server), it would be blacklisted. There are loads of solutions (services, software and appliances/hardware) that block mail from mail servers listed in these blacklists. Many of these are implemented at a server level (eg by ISPs, company mail servers etc) even though not all client-based solutions have this feature.

If you are interested in finding software to block spam, you can browse our software section or alternatively take a look at our customizable server-based anti-spam software feature comparison; you can check out the ones listed which have public blacklist support here or else tweak the settings yourself here.

Also, you might be interested in taking a look at our services section (eg managed anti-spam solutions) or our appliances section (machines built specifically to stop spam).

How do I close my open relay?

If you are a system administrator interested in closing down your open relay (if you have an open relay, then you should!), you can check out the great list maintained by MAPS at

Where can I get more technical info on the Simple Mail Transfer Protocol (SMTP)?

You may read the original Request For Comments (RFC 821) by Jonathan B. Postel, August 1982 and the improved version (RFC 2821) by The Internet Society, April 2001. The latter also makes RFC 974 and RFC 1869 obsolete.

For more info on what damage an open relay can do to your business, check out this article: “Is your email server an open relay?