Horizon "Interfaces" (as the Send from/to address is known in Horizon) are your points of contact with the outside world.
For example, for outbound messages you may have a "Sender" email address from which your emails will be sent, and a mobile number from which your SMS messages will be sent. For receiving email replies you may have a different "Reply to" email address, and for receiving SMS text messages you may have an SMS "long number".
All of these are Interfaces. The message sending interfaces are called "Outbound interfaces", and the receiving ones are called "Inbound interfaces".
Changing/add a new interface
To change or add a new interface, go to Settings > Interfaces
You can edit an existing interface or select 'Add new interface' to add a new one
Please get in touch with your account manager if you are unsure of any changes, as this could have dire effects on your account. Especially if you want to add a new email interface that isn't a horizon.com address (as we need permission to send from other addresses) and this could cause your emails to be blocked.