My Exchange 2007 does not accept emails from POPcon and answers "530 5.7.1 Client was not authenticated"!

The problem is caused by the default authentication settings of the Exchange 2007 SMTP receive connector. By default the receive connector only allows authenticated transfers. But since POPcon simulates incomming email from remote servers it can not authenticate itself to Exchange. You can change the authentication settings with the Exchange Management Shell command

Get-ReceiveConnector -identity *Default* | set-ReceiveConnector -permissiongroups:"ExchangeUsers,ExchangeServers,ExchangeLegacyServers,AnonymousUsers"

This will allow email transfers from POPcon to Exchange without prior authentication again.

Another way is using the Exchange Management Console: To find receive connectors in the Exchange Management Console, navigate to Server Configuration -> Hub Transport. Then select your server and you'll see the Receive Connectors tab below. The "Default" receive connector on Hub is configured for other Exchange servers to authenticate, but it does not accept anonymous email by default as it needs to accept actual SMTP emails from outside/POPcon.

We can find these configuration settings under Server Configuration, Hub Transport, Receive Connectors, Properties of the Default Receive Connector:

Exchange 2007 Receive Connectors

Open the Permission Groups tab and check the Anonymous Users option to allow POPcon to connect to this receive connector.

Exchange 2007 Receive Connector Permission Groups