On various dialogs and security features throughout MDaemon you will see options that allow you to choose whether or not "Trusted Hosts" or "Trusted Domains" will be exceptions to or exempt from those options. The hosts you list on this screen are the ones to which those options refer.
Trusted hosts
This is the list of hosts that will be exempt from certain designated security options.
New trusted host
Enter a new host to be added to the Trusted hosts list.
Comment
Use this for any comment text about an entry.
Add
Click this button to add the new domain to the Trusted hosts list.
Remove
Click this button to remove the selected entries from the Trusted hosts list.