By default, Exchange use the local name for the ReceiveConnecter, which could be problematic with anti-spam measures on most MTA on the Internet. This is recommended to use your reverse dns name (FQDN), which in most case, should or must be the MX record.
To do so, open the Exchange Management Shell and use the following command to get the current identity name (Internet Receive Connector) :
Get-ReceiveConnector
And then, use the following command to change the ReceiveConnector to the correct value :
Set-ReceiveConnector -Identity "Internet Receive Connector" -Banner "220 Banner Text here"
You can also set a timeout value by adding this to the previous command :
-ConnectionTimeout 00:15:00