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.