Django reusable application with linked FK?
Often it's ok to import models between apps.  This just creates a dependency, something many apps have.  Of course it's more flexible to have your app be independently pluggable, but  the important thing is that you document any dependencies for anyone else trying to use your app(s).  
If you really want your app to be pluggable, consider reorganizing your app.  Simplicity is good, but going over-overboard and insisting on strict, literal adherence to principles can get in the way of functionality.  
(Without specific details of your app, this is just speculation, but since all the apps you describe  revolve around Contacts, it seems like they could simply be repackaged into the same app with unsubscribe as boolean field in contacts and a view to set the attribute.  And depending on what exactly you want to do with Email, something similar)
I have the following scenario.
I have an existing application called ‘Contacts’ with it's model I have number and name.
I want to create a new application called ‘unsubscribe’ and I want to make it reusable. 
This is my issue:
In the new app called unsubscribe it's model will need a foreign key relationg to the contact number. This now means that it is now tied to ‘contacts’ and I cannot use it for say my email app. How does Django deal with this from a reusable point of view?