Answer: In Exchange Server 2010 the receive connector represent a logical gateway which all inbound messages are received. The receive conector is configured on the Hub or the Edge server role.
In order to receive messages from the internet you have to configure a receive conector, by default only internal messages receive conector is configured.
This article will show how to create a receive conector on Exchange Server 2010:
Open EMC -> Server configuration -> Hub Transport
In the lower panel right click on a blank area and select New Receive Connector
In the New Receive Connector wizard name the conector and select Internet for the Use.
Select the IP address the will use to receive mail.
After you create the connector, you can start redirecting SMTP traffic from your firewall the Exchange server.