One of my clients reported this problem
On investigation, it was found that this error was returned only if the server was telnet from outside the office network
The usual service banner was coming up if the exchange server was telnet from inside the office.
This led me to check the Hub transport on the server, where I found that someone had deleted the internet receive connector
The default SBS connector only facilitates email to be received from other exchange servers inside the network.
It is due to this connector that the service banner was showing ok internally.
It is the internet connector which is responsible of receiving emails from WWW
So I created the connector and below are the settings:
Note that I have allowed upto 40MB of attachments and the internal network subnet is: 192.168.100.0/24