The SMTP banner is the SMTP connection response that a remote SMTP messaging server receives after it connects to a Receive connector that’s configured on a computer running Microsoft Exchange Server 2010/2013.

By default, Exchange will set the smtp banner to the local server name. However, this can cause serious issue when communicating with other mail servers on the Internet.

This is the default response received by a remote SMTP messaging server after it connects to the Receive connector:
220 <Servername> Microsoft ESMTP MAIL service ready at <RegionalDay-Date-24HourTimeFormat> <RegionalTimeZoneOffset>

Use the Exchange Management Shell to modify the SMTP banner on the Receive connector:

Get the current ReceiveConnector name	Get-ReceiveConnector This should return something like this: EXCHANGE_SERVER_NAME\Default EXCHANGE_SERVER_NAME {0.0.0.0:25} True
To rename the banner matching your mail server fully qualified domain name (replace “mail.domain.com” by yours. The change will be immediate.	Set-ReceiveConnector -Identity “Default EXCHANGE_SERVER_NAME” -Banner “220 mail.domain.com“