You may use the PropertyAccessor of Outlook's object model like this:
Set oPA = oMail.PropertyAccessor
oPA.SetProperty myProp, myValue
I just don't know the value for the 'restricted' property but if you first create a mail with this property and then read the list of assigned properties, you'll find the required value. Maybe it is combined within the 'flags' property - in this case it may be a little more difficult to separate it.
Good luck!