To accept inbound mail, you need to configure a receive connector within Microsoft Exchange. This article covers Microsoft Exchange 2010, 2013, and 2016.
Use the EAC to create an Internet Receive connector on Mailbox servers
Use the EAC to create a Receive connector to receive messages from the Internet.
Use the EMC to create a Receive Connector
To verify that you have successfully created a Receive connector to receive messages from the Internet, test that you can send mail from an outside source and one of your users can receive it. If you can receive mail, you know that the configuration worked successfully.