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Open Relay - How To Avoid Being One.

Having an email server that relays unsolicited email from any source is a fast way to get your entire IP block into the email blacklists. This guide will explain what an open relay is is and help you determine if your email server is an open relay.

What Is An Open Relay?

To understand what an open relay is it may help to think of an email server as a cardboard box. This box has two holes. One is where email comes in and the other is where email goes out. We'll call this "our box."

Under normal conditions, our box will have been set up to only allow email to come in from us. The email we put in then comes out the other end and is sent to its destination.

When our box is not set up properly it lets untrusted outsiders send their email through it. When these untrusted senders' emails get to their destination it’s going to appear as if their emails originated from our box. As you can imagine this can be very harmful for our good reputation as senders of good email.

To go into more detail, an email server usually only allows incoming email that is destined to an address on the server. An email that comes into domain1.com must be addressed to an address, such as user1@domain1.com, in order to be accepted. An email addressed to user1@domain2.com would be refused. But when the server is running as an open relay it will accept and forward messages destined to any email address.

The image below illustrates the difference in how a secured server and an open relay function.

Email Open Relay

How do I tell if I’m an Open Relay?

There are several ways to check this. But the easiest way by far is to use the Unlock The Inbox Open Relay Tool. It will attempt to relay a message through your email server and report whether it was successful or not.

My server is an Open Relay. Now what?

You should consult the documenation that came with your email server to find the appropriate steps to take to turn relaying off. You can come back to the Unlock The Inbox Open Relay Tool to retest your servers once you've made the changes.

Source: Unlock The Inbox
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