Exchange 2010 can send email from POP 3 using anonymous account without password

To prevent the spam or anonymous using your domain email :

1.Open the Exchange Management Console and go to Server Configuration - Hub Transport and create a new Receive Connector.

 2. Right click and go to properties on your new Receive Connector. Go to Network tab and  change the IP address with 0.0.0.0/32 or 0.0.0.0-255.255.255.255 and add your exchange IP Address,

3. Go to Authentication tab and tick Transport Layer Security (TLS) and Externally Secured.

 4. Go to Persmission Group tab and tick Exchange servers only and then click Apply and Ok.