— 3 min read


I have built and released an open-source email server in the past for testing send rates and speeds, this project was called SimpleMTA and is available here.

Recently I have rebuilt this project for an internal project at work using the Tornado framework. Sadly this project as a whole cannot be released but a version of this code will be released in the near future.

Until that is released I have launched a new service called blackhole.io

What is blackhole.io?

blackhole.io is a completely open mail relay that forgets anything that is sent to it, meaning there is no auth requirements and no storage of email data within the service. Literally anyone can send anything to it and have it never get delivered.

You can even send commands out of order, meaning you can call the DATA command without ever using HELO, MAIL FROM or RCPT TO.

What can I use it for?

Honestly, anything. I have spoken to a few people and have come up with a few use cases but I’m sure there will be more:

1. You want to work on production data but do not want to accidentally send an email to real users

This is bit of a nasty situation to be in but, it happens every so often. Sure, you can spend time anonymising all of your user data (and that is the best thing to do), but sometimes you don’t have time or you cannot be 100% sure that you have got it all.

If you application is written in such a way that changing it’s SMTP settings will work across the whole application then you can simply point the SMTP host to “blackhole.io” and ensure that any outgoing emails will go to the blackhole and thus, never reach real people.

2. Testing the speed of your email sends

Maybe you’re writing your own mail platform and want to see how fast you can really send email. blackhole.io is a very fast server and, because it does no real validation and has no data storage it has an extremely low CPU and memory footprint. You can fire a vast amount of mail at it and it should not feel it in the slightest.

I have managed to send over 150,000 emails within an 10 minute period and it didn’t feel any stress.

3. Integration tests

This one was pointed out by a colleague of mine - you have a system, you’ve done your basic testing and unit testing but you want to test integration. blackhole.io allows you to do this, you can send all of your test email data at the service to test your SMTP outbound connections.

In a future release of blackhole.io you will be able to create a free account and set-up logging of mail, allowing you to log the from address and subject line and possibly some custom header data, but this feature is not available currently.


I hope some people will find this service useful, let me know if you use it and what you think of it.


Anarchist. Pessimist. Bipolar. Hacker. Hyperpolyglot. Musician. Ex-(semi-)pro gamer. They/Them.

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