This article provides brief information and steps to resolve the following error when sending email from Serv-U:
550.571 - Unable to relay
This can be seen if telnet is used to the server on port 25 and initiate unauthenticated SMTP communications.
Serv-U version 12.x or later
To permit a non-Exchange server to relay mail we can create a new Receive Connector on the Hub Transport server.
1. Launch the Exchange Management Console and go to Server Management > Hub Transport.
2. Select the Hub Transport server you wish to create the new Receive Connector on.
3. From the Actions pane of the console, select New Receive Connector.
4. Name the new connector as Relay and click Next to continue.
Note: You can leave the local network settings as is, or optionally you can use a dedicated IP address for this connector if one has already been allocated to the server. Using dedicated IP addresses for each connector is sometimes required if you need to create connectors with different authentication settings, but for a general relay connector, it is not necessary to change it.
5. Highlight the default IP range in the remote network settings and click the red X to delete it.
6. Click Add and enter the IP address of the server you want to allow to relay through the Exchange server. Click OK to add it and then Next to continue.
7. Click New to complete the wizard.
Note: The Receive Connector has now been created but is not yet ready to allow the server to relay through it.
8. Go back to the Exchange Management Console, right-click the newly created Receive Connector and select Properties.
9. Click the Permission Groups tab and select the Exchange Servers box.
10. Click the Authentication Tab and select the Externally Secured box.
11. Apply the changes.
The Receive Connector should now be ready for the server to relay through.