if you are still getting smtp banner error here is a good blog to check for 220 smtp banner response and possibly correct the problem. It is the response received by a remote server after it connects to the receive connector of an Exchange 2010 Hub or Edge server.
to check your 220 banner in power shell:
Get-ReceiveConnector -identity "<servername>\<receive connector name>" | fl banner
Most likely your response will be blank:
to set the banner response (mail.domain.com) to match your external records:
Set-ReceiveConnector –identity “ServerConnectorname” –Banner “220 banner text”http://www.howexchangeworks.com/2011/06 ... -2010.html
This is a general configuration and may not apply to your environment. Please do your research before running this command in your environment. Results may vary. Make sure you have a back out plan before running the power shell command. Remember measure twice cut once.