The Fortimail server is used to scan the mail before passing it onto a server to collect for mail. This server can change to match new company requirements, hosting movements between companies or upgrades within the same company.  Therefore the smtp server the fortimail forwards the mail to will need to be changed, this is done from the site admin control panel, under System, Mail Settings.

A secondary server can also be specified should the primary refuse the connection.  You are also able to change the port used to receive mail on for the receiving server.