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How to check if your Exchange Server is open for relay.

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You would notice in your Exchange System Manager SMTP Queues there would be many messages going to unknown domains from unknown senders.

So, it is best to check is your server open for relay.

Step 1. Is to run the basic telnet command on your EXCHANGE SERVER (You may want to replace localhost with your server name or its IP Address)Telnet Command
Step 2. When you process the telnet command, you would see the following result.
Out put of Telnet Command
Step 3 Need to type in EHLO and hit enter OR type in HELO and hit enter to see the following result (Keep in mind the number of SMTP Verbs listed in this output - it has to be the same on all Exchange servers)

EHLO
Step 4. Next is to process the Mail from command (I choose to use administrator@yahoo you may choose to write using any other third-party domain which is not local to Exchange).

You may choose different variants of mail from commands as per Microsoft KB 304897

MAIL FROM
Step 5. Now you would need to type in RCPT TO (this would be a recipient outside your Exchange Organization)

RCPT TO
Notice the result of the command "Unable to relay for recipient@domain.com" - This would mean that your server is not open for relay. However, if you do not get this result and get a result instead you find the cursor spacing in the next line - this means that the Exchange is waiting for you to relay. This means you are open to relay.

So, where to check if your server is open for relay ?

Open Exchange System Manager.
=> Organization
=> Administrative Groups
=> Your A.G
=> Servers
=> Your Exchange Server
=> Protocols
=> SMTP
=> Default SMTP Virtual Server properties
Access tab => Relay tab.
(Check the basic setting as the pic attached)

SMTP Virtual Server
And one more location


Open Exchange System Manager.
=> Organization
=> Administrative Groups
=> Your A.G
=> Routing Groups
=> Connectors
=> SMTP Connector properties
=> Address Space tab.

(Check the basic setting as picture mentioned - the "Allow messages to be relayed to these domains" should be unchecked)

SMTP Connector
Going through these steps, you should be able to check if you are open for relay and curb the relaying.
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