When a contact is first created, a list of Virtual Mailing Preferences will appear by default. These are displayed with a grey tick The Preference Type at the end of the row will have the word Virtual.
If the contact is setup as Do Not Mail they will have a Grey cross
In this scenario, this contact will receive any preferences with a grey tick, if included in a mailing that does not require explicit preferences eg a green tick. They will be sent to the main address on the contact details form (or their main email address) depending on the type of communication.