You can't add a specific host name or IP address as a smart host if the Send connector has a "-" value specified as a smarthost. ...

You can't add a specific host name or IP address as a smart host if the Send connector has a "--" value specified as a smarthost. This Send connector is managed by the Microsoft Exchange EdgeSync service to deliver inbound e-mail, and the value "--" refers to all Mailbox servers in the Active Directory site to which this Edge server is subscribed. Please refer to the EdgeSync documentation before you change the smart host values.