BIS is usually a individual or small business and they use the internet to get their mail and for some settings. If you are BIS you have a login to a web site. The default signature is there and would contain the "Sent from..." verbiage.
BES is usually an employee of a larger or at least more technically aware company. They get their mail via their company server and I believe that is where they will find their signature (but not sure about BES).